Move doc string generator into separate module

Move the doc string generator for creating function signatures
into "gi.docstring". This includes a new API for getting and
setting the doc string creation functions:

gi.docstring.get_doc_string_generator
gi.docstring.set_doc_string_generator
gi.docstring.generate_doc_string

Beyond adding the ability for custom doc string generators,
this API is a necessary step for adding lazy __doc__
attribute access for optimization.

https://bugzilla.gnome.org/show_bug.cgi?id=704037