Add span-based Guid.{Try}Parse{Exact} methods (#14062)

This commit does two things:
- Changes the existing Guid.{Try}Parse{Exact} methods to work in terms of spans rather than strings.
- Then adds new overloads that accept spans and that uses the same underlying span-based support.

Due to the first change, performance actually improves for several of the guid formats, in particular due to substring allocations that are no longer incurred.  For example, this program:
```C#
using System;
using System.Diagnostics;

class Program
{
    static void Main()
    {
        var sw = new Stopwatch();
        const int Iters = 1000000;

        Console.WriteLine("Pattern\tBytes\tTime");
        foreach (string pattern in new[] { "D", "B", "P", "N", "X" })
        {
            string input = Guid.NewGuid().ToString(pattern);
            Guid.Parse(input);

            long a = GC.GetAllocatedBytesForCurrentThread();
            sw.Restart();
            for (int i = 0; i < Iters; i++) Guid.Parse(input);
            sw.Stop();
            a = GC.GetAllocatedBytesForCurrentThread() - a;

            Console.WriteLine($"\"{pattern}\"\t{a / Iters}\t{(int)(sw.Elapsed.TotalMilliseconds / Iters * 1000000)}ns");
        }
    }
}
```
on my machine previously output:
```
Pattern Bytes   Time
"D"     0       218ns
"B"     0       217ns
"P"     0       210ns
"N"     168     388ns
"X"     744     765ns
```
and with this change outputs:
```
Pattern Bytes   Time
"D"     0       213ns
"B"     0       192ns
"P"     0       196ns
"N"     0       372ns
"X"     0       577ns
```

src/mscorlib/shared/System/Convert.cs

@@ -2276,7 +2276,13 @@ public static byte ToByte(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            int r = ParseNumbers.StringToInt(value, fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsUnsigned);
+
+            if (value == null)
+            {
+                return 0;
+            }
+
+            int r = ParseNumbers.StringToInt(value.AsReadOnlySpan(), fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsUnsigned);
             if (r < Byte.MinValue || r > Byte.MaxValue)
                 ThrowByteOverflowException();
             return (byte)r;
@@ -2294,7 +2300,13 @@ public static sbyte ToSByte(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            int r = ParseNumbers.StringToInt(value, fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsI1);
+
+            if (value == null)
+            {
+                return 0;
+            }
+
+            int r = ParseNumbers.StringToInt(value.AsReadOnlySpan(), fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsI1);
             if (fromBase != 10 && r <= Byte.MaxValue)
                 return (sbyte)r;
 
@@ -2314,7 +2326,13 @@ public static short ToInt16(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            int r = ParseNumbers.StringToInt(value, fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsI2);
+
+            if (value == null)
+            {
+                return 0;
+            }
+
+            int r = ParseNumbers.StringToInt(value.AsReadOnlySpan(), fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsI2);
             if (fromBase != 10 && r <= UInt16.MaxValue)
                 return (short)r;
 
@@ -2335,7 +2353,13 @@ public static ushort ToUInt16(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            int r = ParseNumbers.StringToInt(value, fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsUnsigned);
+
+            if (value == null)
+            {
+                return 0;
+            }
+
+            int r = ParseNumbers.StringToInt(value.AsReadOnlySpan(), fromBase, ParseNumbers.IsTight | ParseNumbers.TreatAsUnsigned);
             if (r < UInt16.MinValue || r > UInt16.MaxValue)
                 ThrowUInt16OverflowException();
             return (ushort)r;
@@ -2352,7 +2376,9 @@ public static int ToInt32(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            return ParseNumbers.StringToInt(value, fromBase, ParseNumbers.IsTight);
+            return value != null ?
+                ParseNumbers.StringToInt(value.AsReadOnlySpan(), fromBase, ParseNumbers.IsTight) :
+                0;
         }
 
         // Parses value in base fromBase.  fromBase can only
@@ -2367,7 +2393,9 @@ public static uint ToUInt32(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            return (uint)ParseNumbers.StringToInt(value, fromBase, ParseNumbers.TreatAsUnsigned | ParseNumbers.IsTight);
+            return value != null ?
+                (uint)ParseNumbers.StringToInt(value.AsReadOnlySpan(), fromBase, ParseNumbers.TreatAsUnsigned | ParseNumbers.IsTight) :
+                0;
         }
 
         // Parses value in base fromBase.  fromBase can only
@@ -2381,7 +2409,9 @@ public static long ToInt64(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            return ParseNumbers.StringToLong(value, fromBase, ParseNumbers.IsTight);
+            return value != null ?
+                ParseNumbers.StringToLong(value.AsReadOnlySpan(), fromBase, ParseNumbers.IsTight) :
+                0;
         }
 
         // Parses value in base fromBase.  fromBase can only
@@ -2396,7 +2426,9 @@ public static ulong ToUInt64(String value, int fromBase)
                 throw new ArgumentException(SR.Arg_InvalidBase);
             }
             Contract.EndContractBlock();
-            return (ulong)ParseNumbers.StringToLong(value, fromBase, ParseNumbers.TreatAsUnsigned | ParseNumbers.IsTight);
+            return value != null ?
+                (ulong)ParseNumbers.StringToLong(value.AsReadOnlySpan(), fromBase, ParseNumbers.TreatAsUnsigned | ParseNumbers.IsTight) :
+                0;
         }
 
         // Convert the byte value to a string in base fromBase