This removes the old unused gjs_parse_args(), and moves
gjs_parse_call_args() into new files jsapi-util-args.cpp and
jsapi-util-args.h.

In order to ensure that JSObjects unpacked from gjs_parse_call_args()
land in GC roots, we need to add some type safety to its variable
arguments. Instead of accepting a JSObject** pointer for the "o" format
character, it must accept a JS::MutableHandleObject.

We can do this (and make the whole thing type safe as well) by using C++
templates instead of C varargs. Now we can issue a compile-time error
when an unknown type is passed in as a return location for an argument,
in particular JSObject**.

This also fixes some undefined behaviour: the old signature of
gjs_parse_call_args() placed the varargs parameter right after the
JS::CallArgs& parameter, and using a reference parameter in va_start() is
undefined. Here's a good explanation of what can go wrong:
http://stackoverflow.com/a/222288/172999

https://bugzilla.gnome.org/show_bug.cgi?id=742249