Add benchmarks

Clarification and motivation

It's a good practice to have benchmarks for your code. And since our code is mostly pure, adding benchmarks to it should be quite easy. I propose to start with benchmarking the following functionality:

1. Parsing contracts.
2. Typechecking contracts.
3. Interpreting contracts in isolation (i. e. without updating GState and potentially calling other contracts).

Acceptance criteria

1. There should be a benchmark which reports how much time it takes to parse some contracts, typecheck some contracts and interpret some contracts in isolation.
2. CI should execute this benchmark and display its result somehow.