CN: Remove base type checks in check.ml

[dc-mak]

I'm fairly sure they are all redundant/should be caught by wellTyped.ml. Should also remove ensure_base_type and ensure_bits from wellTyped_intf.ml too.

[dc-mak]

Ugh, depends on #272 which is almost definitely not handling the integer promotions correctly in the presence of bit vectors...

[cp526]

Let's wait with this one for now. (1) It's nice if check.ml is self-contained, in that it includes all the type checks necessary for soundness without depending on another module (of course it currently does depend on wellTyped having sufficiently type-annotated the Mucore code), but more importantly (2) I want to explore ownership types, and then resource checking and basetype checking would be more intertwined; removing basetype checks from check.ml would undo work that I'd have to then re-do.