srml-contract: Contract refactors (#2924)

* srml-contract: Refactor away unnecessary Option.

* srml-contract: Add assertion to gas_left test.

* srml-contract: Refactor try_evict_or_and_pay_rent to make tests pass.

* srml-contract: Add tests and comments for bugs in rent payment logic.

* srml-contract: Minor cleanup using GasMeter constructor.

* Bump node runtime impl version.