Extract nested classes to separate files

[FroggieFrog]

This idea for this PR is based on #60, but it just extracts all nested classes to their own files (CTRL + . and "Move to xx.cs"). this is common best pratice in the .NET world. Also all files were formatted via Edit->Advanced->Format Document.

Hopefully it makes it easier to add the performance improvements from #60 and future ones.

[enemaerke]

Finally had a look at this. Beyond the move to separate files, I think you also moved some classes into other namespaces (Implementations, Abstractions, Extensions)? I am personally not a fan of such a separation but more critical is that it'll break backwards compatibility if we move existing classes to new namespaces. I would prefer a fully flat namespace.