gutils: clean up bit funcs inlining mess

gutils.h and gutils.c define three utility functions as inlines that are
also exported via the ABI.  This is done via complicated G_INLINE_FUNC
and G_IMPLEMENT_INLINES logic.

In order to be able to remove this mess, we create a another convoluted
but slightly cleaner approach: write straight-up inline versions of the
functions named _impl() in the header.  Define macros with the "public"
function names that call these inlines.  From the .c file, export the
ABI versions of these functions, implemented using the _impl() version.

https://bugzilla.gnome.org/show_bug.cgi?id=757374