cache refactoring: Use consistent prefix for arg cache functions

Use "pygi_arg_cache" as the prefix for arg cache memory related functions.

https://bugzilla.gnome.org/show_bug.cgi?id=709700
cache refactoring: Add comments to arg cache setup functions

https://bugzilla.gnome.org/show_bug.cgi?id=709700
Restore pygobject_version API needed for pygobject.h

Add gi._gobject.pygobject_version which was removed with commit:
https://git.gnome.org/browse/pygobject/commit/?id=2624bd2b
This is needed for pygobject.h to function properly.