Additional Memory<byte> support

[cocowalla]

#68 added some support for RecyclableMemoryStream to read and write Span<byte>/Memory<byte>. It would be good to also add support for creating a RecyclableMemoryStream from an existing Memory<byte>.

RecyclableMemoryStream.GetStream already has an overload that takes an existing byte[] buffer:

MemoryStream GetStream(string tag, byte[] buffer, int offset, int count)

So the suggestion here is to add a new overload that takes an existing Memory<byte> buffer:

MemoryStream GetStream(string tag, Memory<byte> buffer)

[benmwatson]

Sounds reasonable. I will try to get to it soon-ish. I'm doing a huge .NET Core migration this week and next, but after that I can relax.