arg: Allocate hash values that don't fit in pointers

For GHashTables with value types that can't be stuffed into pointers, we
use heap-allocated values instead. Since the types we are concerned with
here are all primitive types, they are always copied, so the hash table
can always own the values, and they can be freed unconditionally when the
hash table GIArgument is released.

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