[mypyc] Foundational work to help support native ints

[JukkaL]

Some IR and codegen changes that help with native int support.

This was split off from a branch with a working implementation of
native ints to make reviewing easier. Some tests and primitives
are missing here and I will include them in follow-up PRs.

Summary of major changes below.

1. Allow ambiguous error returns from functions. Since all values
   of i64 values are valid return values, none can be reserved for
   errors. The approach here is to have the error value overlap a
   valid value, and use PyErr_Occurred() as a secondary check
   to make sure it actually was an error.

2. Add Extend op which extends a value to a larger integer
   type with either zero or sign extension.

3. Improve subtype checking with native int types.

4. Fill in other minor gaps in IR and codegen support for native
   ints.

Work on mypyc/mypyc#837.

[ilevkivskyi]

This concept is a bit counterintuitive, so maybe specifically give an example of fixed ints here? Also do I understand correctly that the only kind of error that can happen is an overflow (zero division could only come from float)?