srml-contract: Reuse instrumented code

[pepyakin]

Before execution smart-contract code must be instrumented (e.g. with gas metering). Now, it is performed each time before execution of smart-contract but this is wasteful since the result of instrumentation is almost always the same. The caveat is that instrumentation depends on gas schedule (which can be changed by governance procedures), and we have to make sure that the instrumented code was created with a matching gas schedule.