Report boxed math warning on function that boxes primitive return value

With the new :warn-on-boxed (CLJ-1325), these examples do not report a boxed math warning although they each do boxing:

Cause: emitBoxReturn has a hard-coded call to box a prim return value.

Solution: If *unchecked-math* is set to :warn-on-boxed, emit a warning on boxing of primitive numeric return types.

Patch: