De-mangle Rust function names

[gregtatum]

[mstange]

For official builds of Firefox / Nightly, bug 1311498 would gives us this for free.

Not sure what to for local builds on Linux and Mac.

For local builds on Linux, the demangling is currently done by nm during the symbol dumping because we call nm with the --demangle flag. Apparently nm is getting updated to handle rust automatically in the future; it would be nice to not have to wait for that.
For local builds on Mac, the demangling is done by running c++filt on nm's output, because Mac's nm does not have a --demangle option.
http://searchfox.org/mozilla-central/rev/f55349994fdac101d121b11dac769f3f17fbec4b/browser/components/extensions/ext-geckoProfiler.js#143-173

[fqueze]

@mstange Is there anything left to do here?

[mstange]

Bug 1474871 fixed this for official builds and PR #1203 fixed it for local builds. We're done here :-)