Add support for tracking names used in Scala source files.

Tracking of used names is a component needed by the name hashing
algorithm. The extraction and storage of used names is active only when
`AnalysisCallback.nameHashing` flag is enabled and it's disabled by
default.

This change constists of two parts:

  1. Modification of Relations to include a new `names` relation
     that allows us to track used names in Scala source files
  2. Implementation of logic that extracts used names from Scala
     compilation units (that correspond to Scala source files)

The first part is straightforward: add standard set of methods in
Relations (along with their implementation) and update the logic which
serializes and deserializes Relations.

The second part is implemented as tree walk that collects all symbols
associated with trees. For each symbol we extract a simple, decoded name
and add it to a set of extracted names. Check documentation of
`ExtractUsedNames` for discussion of implementation details.

The `ExtractUsedNames` comes with unit tests grouped in
`ExtractUsedNamesSpecification`. Check that class for details.

Given the fact that we fork while running tests in `compiler-interface`
subproject and tests are ran in parallel which involves allocating
multiple Scala compiler instances we had to bump the default memory limit.

This commit contains fixes for gkossakowski/sbt#3, gkossakowski/sbt#5 and
gkossakowski/sbt#6 issues.