More detailed logging of incremental compiler's invalidation logic.

The following events are logged:

  * invalidation of source file due to macro definition
  * inclusion of dependency invalidated by inheritance; we log both
    nodes of dependency edge (dependent and dependency)

The second bullet helps to understand what's going on in case of
complex inheritance hierarchies like in Scala compiler.
Represent api changes as values and cleanup APIChanges class.

The main motivation behind this commit is to reify information about
api changes that incremental compiler considers. We introduce a new
sealed class `APIChange` that has (at the moment) two subtypes:

  * APIChangeDueToMacroDefinition - as the name explains, this represents
    the case where incremental compiler considers an api to be changed
    just because given source file contains a macro definition
  * SourceAPIChange - this represents the case of regular api change;
    at the moment it's just a simple wrapper around value representing
    source file but in the future it will get expanded to contain more
    detailed information about API changes (e.g. collection of changed
    name hashes)

The APIChanges becomes just a collection of APIChange instances.
In particular, I removed `names` field that seems to be a dead code in
incremental compiler. The `NameChanges` class and methods that refer to
it in `SameAPI` has been deprecated.

The Incremental.scala has been adapted to changed signature of APIChanges
class. The `sameSource` method returns representation of APIChange
(if there's one) instead of just simple boolean. One notable change is
that information about APIChanges is pushed deeper into invalidation logic.
This will allow us to treat the APIChangeDueToMacroDefinition case properly
once name hashing scheme arrives.

This commit shouldn't change any behavior and is purely a refactoring.
Merge pull request #941 from gkossakowski/apichanges-refactor

More detailed logging and APIChanges refactoring