Add wrappers for Python implementation functions and methods

This adds some wrappers for Python implementation functions and
methods, and a couple of new constexpr functions to create PyMethodDef
entries.  This provides a few safety benefits:

* The new-style API approach (see previous patch) is implemented by
  the wrapper.  That is, exceptions are caught here and transformed.

* The implementation functions can now return any reasonable type,
  with automatic conversion by the wrapper.

* The function API and the appropriate METH_* flags are handled
  together, avoiding any possible discrepancy.

This approach also means that we can modify the old rule that gdb
calls must be wrapped in a try/catch -- the try/catch is now provided
by the wrapper function, so the implementation can be written in a
more natural way.

Note that while this patch is usable as-is, it is not 100% complete,
in sense that there is still future work to do when converting other
parts of the gdb Python code.  For instance, there should be one more
wrapper for case where a method takes a single argument (though we
probably cannot use METH_O unfortunately).

Acked-By: Tom de Vries <tdevries@suse.de>