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.

substrate/node/runtime/src/lib.rs

@@ -59,7 +59,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 99,
-	impl_version: 103,
+	impl_version: 104,
 	apis: RUNTIME_API_VERSIONS,
 };