fix nativecast test failures

since the built-in slicing does not like our CArray, provider our
own (potentially slicing) multi postcircumfix:<[ ]>.

No idea if this is the right fix. Maybe TimToady++ has a better idea.
Fix tests

turns out that we must not return a Parcel for a single-element slice/array access
Merge branch 'master' of https://github.com/jnthn/zavolaj into cglobal