Fix converting enumerations and flags to arrays

Enumerations and flags are classed types for Vala, not integers, so
they don't fall in the `typeof(G) == typeof(int)` kind of tests. This
leads to using the generic code in which Vala assumes pointer-sized
elements, which is often not true for enumerations and flags.

Add special case for those to use the `int` converters for enumerations
and flags.

This is most generally correct, but not always: the compiler will
likely chose a larger type for a specific enumeration if one of its
value is larger than `int`.  It would be tempting to use the
enumeration's class minimum and maximum values to determine the
appropriate type, but unfortunately the API for this uses int itself,
so doesn't help.

https://bugzilla.gnome.org/show_bug.cgi?id=774669