MVM_nativecall_cast added

MVM_nativecall_cast has been added which provides casting an
OpaquePointer to any type we want. It makes us able to unpack things
conditionally.
fix bogus declaration (copy&pasto)
add box target parameter to nativecallcast

This op now gets a param that says something about the bit size we are
going to read, and also the type we wanna box into. The latter will be
either of Int/Num/Str, when the former is int8/int16/...