Remove needless Aggregator dependency from Class.
Unify interface of all extra processing classes.

So they're all called using the pattern:

    SomeClass.new(@relations).process_all!
Move all processor classes to process/ subdir.

To clean up extra cruft from the main /lib/jsduck dir.
Unify naming of JsDuck::Process::* classes.
Encapsulate most of Aggregator class methods.

Aggregator now only exposes #aggregate and #result methods.  The result
now returns a Hash instead of Array - a change which caused a change in
lots of specs, but we're now better off as the specs rely less on
positional indexing in #result and instead look up classes by name.