Optimize CPU intensitive Compiler methods

This commit contains some performance optimizations in methods
that showed up as very time consuming in the profiler when running
the rake following rake tests:

benchmark:many_modules:run
benchmark:defined_types4:run
benchmark:defined_types:run

CPU consumption with this patch in place is between 70% - 75% of
what it used to be.

Profiling was performed using stackprof
https://github.com/tmm1/stackprof
ok, so three alternatives to maintain uniqueness:

1. Go back to using the |= operator and force an unnecessary creation of an extra array instance.
2. Use a Set instead of an Array.
3. Test using array.include? before adding.

My preference is 3. Thoughts?