Pass an MVMCollectable* to MVM_ASSIGN_REF(), without using casts.

All derivatives of MVMObject and MVMSTable contain a nested MVMCollectable
struct. By using structure member access, we can get a pointer to it without
using a cast. This encourages type safety, and reduces the number of casts
and hence places that might conceal strict aliasing violations.
Get latest NQP, for Moar bump/NativeCall bits.