Pulls out the NMParser work (parsing nm results and turning
them into an ArrayBuffer'd map of addresses to symbols) into
a worker.
For OSX we will still need to do some work to run c++filt in the
background, but this gets us most of the way there. Without a
Subprocess.jsm usable from a worker, we'll have to bounce data
back to the main thread in order to bounce it to the c++filt
worker.
MozReview-Commit-ID: LZi7J1qGpmh
