Add Socket Span/Buffer-based APIs

[stephentoub]

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

• Implement/expose/test span-based methods in System.Net.Sockets in corefx
• Implement/expose/test buffer-based methods in System.Net.Sockets in corefx

namespace System.Net.Sockets
{
    public class Socket
    {
        public int Receive(Span<byte> buffer);
        public int Receive(Span<byte> buffer, SocketFlags flags);
        public int Receive(Span<byte> buffer, SocketFlags socketFlags, out SocketError errorCode);

        public int Send(ReadOnlySpan<byte> buffer);
        public int Send(ReadOnlySpan<byte> buffer, SocketFlags flags);
        public int Send(ReadOnlySpan<byte> buffer, SocketFlags flags, out SocketError errorCode);
        …
    }

    public class SocketTaskExtensions
    {
        public static ValueTask<int> ReceiveAsync(this Socket socket, Buffer<byte> buffer, SocketFlags socketFlags, CancellationToken cancellationToken = default(CancellationToken));
        public static ValueTask<int> SendAsync(this Socket socket, ReadOnlyBuffer<byte> buffer, SocketFlags socketFlags, CancellationToken cancellationToken = default(CancellationToken));
        …
    }

    public class SocketAsyncEventArgs
    {
        public void SetBuffer(Buffer<byte> buffer);
        public Buffer<byte> GetBuffer();
        …
    }
}