Definition liveness & copy trimming

The purpose of these operations is to let the type checker know what definition copies are actually used for type-checking and need to be created, and which can be safely skipped ("trimmed", hence the name renTrimming).

The liveness the scope-checker reports should be a conservative over-approximation of what names are actually used. For example, when an overloaded field/constructor name is resolved, we (have to) mark all of the names in the set as used: not only do we not know which one the type checker will actually pick, but the type checker will ask for all of their getConstInfo when disambiguating.

Marking a name as live is done through the markLiveName function, which is called from resolveName', so any names coming from concrete syntax will already be marked live. However, if potential references to copies are being inserted "synthetically" by the scope checker, they must be referred to by an explicit markLiveName call.

Module names can also be markLiveNamed, and this has the effect of transitively saving every definition having that module as a prefix.