First pass at P6opaque.serialize.
serialize/deserialize for MVMArray.
Improve missing serialization function error.
Poke serialize function into table for P6str.
Merge branch 'master' into static-repr

Conflicts:
	src/6model/reprs/MVMArray.c
	src/6model/reprs/P6opaque.c
	src/6model/reprs/P6str.c
	src/6model/serialization.c