Add IncrementalHash Span-based APIs

[stephentoub]

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

• Implement in System.Security.Cryptography.Algorithms in corefx
• Expose from System.Security.Cryptography.Algorithms contract in corefx
• Add tests to System.Security.Cryptography.Algorithms tests in corefx

namespace System.Security.Cryptography
{
    public sealed class IncrementalHash
    {
        public void AppendData(ReadOnlySpan<byte> data);
        public bool TryGetHashAndReset(Span<byte> destination, out int bytesWritten);
        …
    }
}