Complete libmarshall testing on array types

[spk121]

There are a few test cases left to cover for GArray, GPtrArray, GByteArray, and GBytes in libmarshall

[spk121]

Previously, there was the LATER_FREE mechanism used to free temporary variables created during argument marshalling, and this worked because those temp variables were created using a malloc analog, and the GPtrArray that held objects to be freed called free on all the pointers when it was destroyed. But when adding these array types, there are now temp variables that need to call specific destructors, such as GPtrArray. So the LATER_FREE mechanism will need to be improved. Each pointer needs to be paired with a closure of a free function.

[spk121]

GBytes could just be unpacked as a bytevector, but, it has the added wrinkle that its contents are supposed to be read only. Since there is no such thing as a read-only bytevector on the scheme side, there are a couple of options.

1. Always make a new copy of the contents and return that as a bytevector
2. Keep it as an object, and use methods like g_bytes_unref_to_array unpack its contents
3. Put read-only bytevectors in core Guile

I think I'll roll with option number 2 for now.