Add Guid Span-based APIs

[stephentoub]

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

• Implement in System.Private.CoreLib in coreclr
• Implement in System.Private.CoreLib in corert (if Guid isn't yet "shared" between coreclr and corert)
• Expose from System.Runtime contract in corefx
• Add tests to System.Runtime tests in corefx

namespace System
{
    public struct Guid
    {
        public Guid(ReadOnlySpan<byte> b); // correspond to Guid(byte[] b)

        public bool TryWriteBytes(Span<byte> destination);

        public static Guid Parse(ReadOnlySpan<char> input);
        public static Guid ParseExact(ReadOnlySpan<char> input, string format);

        public static bool TryParse(ReadOnlySpan<char> input, out Guid result);
        public static bool TryParseExact(ReadOnlySpan<char> input, string format, out Guid result);

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

[stephentoub]

The ctor, TryWriteBytes, and TryFormat are done.