Manually clean up paths

Using code:set_path/1 with very large paths is very slow on larger
projects.  On my mid-sized project, it seems to take around .4s per
call.  Emulating the call with direct path removal (using
code:del_path/1) seems to be quite a lot faster.
Merge pull request #344 from evanmcc/pevm/set-path-speedup

Manually clean up paths.