Add BigInteger Span-based APIs

[stephentoub]

Separated out of https://github.com/dotnet/corefx/issues/21281 for tracking purposes.

• Implement in System.Runtime.Numerics
• Expose from System.Runtime.Numerics ref
• Test in System.Runtime.Numerics.Tests

namespace System.Numerics
{
    public struct BigInteger
    {
        public BigInteger(ReadOnlySpan<byte> value);

        public int GetByteCount();
        public bool TryWriteBytes(Span<byte> destination, out int bytesWritten);

        public static BigInteger Parse(ReadOnlySpan<char> value, NumberStyles style = NumberStyles.Integer, IFormatProvider provider = null);
        public static bool TryParse(ReadOnlySpan<char> value, out BigInteger result, NumberStyles style = NumberStyles.Integer, IFormatProvider provider = null);

        public static bool TryFormat(Span<char> destination, out int charsWritten, string format = null, IFormatProvider provider = null);
        …
    }
}

[bazzilic]

When do you think this will reach the big .NET Framework?

[jnm2]

The question on everyone's minds... (at least until .NET Core 3 comes out!)