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

• Implement span-based APIs in System.Private.CoreLib in coreclr
• Implement buffer-based APIs in System.Private.CoreLib in coreclr
• Implement span-based APIs in System.Private.CoreLib in corert (if Stream isn't yet in "shared")
• Implement buffer-based APIs in System.Private.CoreLib in corert (if Stream isn't yet in "shared")
• Expose span-based APIs from System.Runtime contract in corefx
• Expose buffer-based APIs from System.Runtime contract in corefx
• Add span-based APIs tests to System.Runtime tests in corefx
• Add buffer-based APIs tests to System.Runtime tests in corefx

namespace System.IO
{
    public class Stream
    {
        public virtual int Read(Span<byte> destination);
        public virtual ValueTask<int> ReadAsync(Buffer<byte> destination, CancellationToken cancellationToken = default(CancellationToken));

        public virtual void Write(ReadOnlySpan<byte> source);
        public virtual Task WriteAsync(ReadOnlyBuffer<byte> source, CancellationToken cancellationToken = default(CancellationToken));
        …
    }
}