Added All Span APIs to Stream Polyfill

[KrzysztofCwalina]

I added:

public static async ValueTask<int> ReadAsync(this Stream stream, Memory<byte> buffer, CancellationToken cancellationToken = default)
public static Task WriteAsync(this Stream stream, ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
public static void Write(this Stream stream, ReadOnlySpan<byte> buffer)