Add MatrixWriter overload for sequences of unmanaged types

[glopesdev]

This PR expands the compatibility of MatrixWriter to any arbitrary sequence of unmanaged types by taking advantage of Buffer.MemoryCopy to directly marshal the value type into a binary stream.

The new overload signature is:

public IObservable<TElement> Process<TElement>(IObservable<TElement> source) where TElement : unmanaged

Buffer.MemoryCopy is compatible across both .NET Framework and .NET 5+ but we might consider later improving the implementation for .NET 5+ by taking advantage of new memory copy performance primitives.

Fixes #1578

[jonnew]

yay!!!!