clr_loader is based on CFFI and allows loading pythonnet on different
.NET runtime implementations, currently .NET Framework, .NET Core and
Mono. Apart from dropping the respective old code, this requires the following
changes:
- Move libpython discovery into Python by vendoring and adjusting
   `find_libpython`
- Adjust the GIL handling in the startup code as CFFI releases the GIL when
   calling the external function
- Remove the intermittent `configure` command as it is not required
   anymore
- Adjust a few test-cases
- Remove `_AtExit`

Due to issues with the reference counts, the `atexit` callback is
currently essentially a no-op until these are fixed.