Emit compiler warning/error if a permission specifies unused method

[fyrchik]

See nspcc-dev/neofs-contract#265
We can do this if all callsites have constant method names. This should cover 99% of our cases.

Depending on the implementation of neo-project/neo#2756, these warnings could become even more important.

[roman-khimov]

Not sure about this one. When we see a call with clear hash/method combination and can't find a matching permission it's 100% accurate easily detectable error, but it's not the same the other way around. If "all callsites have constant method names" maybe, but even then we have */method permissions that can be too broad in some scenarios.