arg: Set hash functions according to GI type

Previously it was assumed that all GHashTable keys were strings when
constructing a hash table from a JS object. This causes a segfault when
you use, for example, GINT_TO_POINTER(-1) as a key, since g_str_hash will
try to dereference the pointer.

Sadly this does not fix any tests by itself, but it is a prerequisite.

https://bugzilla.gnome.org/show_bug.cgi?id=773763