Gas limits for calls

[xermicus]

Gas does not correspond to ref_time exactly as semantically equivalent code can exhibit totally different gas usage on EVM vs. ref_time usage on PVM.

Right now we ignore it, making all calls unconstrained. But those gas limits are a security feature.

Possible solutions:

1. Scale the supplied gas limit with a fixed factor
2. Treat the supplied gas as relative value relative to the EVM block gas limit and convert it to the corresponding relative ref_time limit. Drawback is that the compiler (i.e. the user) needs to know the absolute ref_time limit of the target blockchain. It also largely disconnects the semantic meaning of the value should the block limits in either chain change.

Also add a warning whenever a call with gas limit != 0 is compiled.