Detect function's effects

[anton-trunov]

Useful, for instance, for static analysis: nowarp/misti#88.

Actually, we need to track more than purity, but effects:

• contract state reads,
• contract state writes,
• blockchain state reads (e.g. time),
• sending messages,
• randomness.

For instance, getters can have the following set of effects: contract state reads, blockchain state reads, randomness.
Const-eval (comptime) functions must be (observationally) pure, i.e. those can only modify some internal state.

Also, showing those effects in the compilation report is important for contract audits.