Split out expressions/evaluators from translate.py

[zbanks]

This PR has two parts:

• The first part moves functions from translate.py into a new evaluate.py file verbatim, in the same order they originally appeared.
  • Exception: CarryBit.add_to & CarryBit.sub_from were rewritten as carry_add_to and carry_sub_from and placed at the end of the file.
• The second part moves Expressions & Statements into a new expression.py file, also in the same order they originally appeared.
  • Exception: StackInfoBase was added as an interface, which StackInfo then implements. (I think StackInfo could be refactored to avoid this inheritance?)
  • I also sorted the imports alphabetically.

Overall, I know this is a big PR, but it splits translate.py up into 3 files (each >1k lines) which I think should help with maintenance?

[simonlindholm]

Sorry to be slow to get to this (and to totally bit rot it with by phi refactoring...) I think I like the first part, it feels like a relatively clean conceptual split. The second I'm a bit more torn about; it's probably good but I need to digest it first... I'm a bit cautious about the split given my vim grep-based workflow, but I think I'll manage. (And I'm not planning similarly invasive changes as my phi work any time soon.)

[zbanks]

I'm going to close this PR then, to preserve a record of the two parts if we want to revisit this later. The first part I re-did & moved to #237 .