Added assigns clauses for loops

1. Renamed cprover_contract_assigns_opt to cprover_contract_assigns as it did not have unit production.
2. Created cprover_contract_assigns_opt that supports unit productions
3. Added cprover_contract_assigns_opt to iteration_statement.
4. Renamed loop_invariant_opt to cprover_contract_invariant_opt for consistency.
5. Renamed loop_decreases_opt to cprover_contract_decreases_opt for consistency.

This adds support for parsing assigns clauses for loops.

Author: Aalok Thakkar