Add Encoding Span-based APIs

[stephentoub]

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

• Implement in System.Private.CoreLib in coreclr ("shared" with corert)
• Expose from System.Text.Encoding contract in corefx
• Add tests to System.Text.Encoding tests in corefx

namespace System.Text
{
    public class Encoding
    {
        public virtual int GetByteCount(ReadOnlySpan<char> chars);
        public virtual int GetBytes(ReadOnlySpan<char> chars, Span<byte> bytes);

        public virtual int GetCharCount(ReadOnlySpan<byte> bytes);
        public virtual int GetChars(ReadOnlySpan<byte> bytes, Span<char> chars);

        public string GetString(ReadOnlySpan<byte> bytes);

        public virtual ReadOnlySpan<byte> Preamble { get; }
        …
    }
}