Allow CSE optimization to cross between future and past time

[aaurandt]

Currently, past and future time specs are kept completely separate within C2PO. This means the even simple instructions like loading an atomic are duplicated for both future and past time.

One possible solution would be to

• Have C2PO still check for mixed time formulas in type_check.py to prohibit users from creating these formulas
• Have past and future specifications be represented as a single data structure in the rest of C2PO to allow for CSE to apply to both