Fix Rust symbols in collapse-dtrace and collapse-perf

[jasonrhansen]

This fixes partially demangled Rust function names like we did with collapse-sample.

#132 was already meant to fix #87, but it required dtrace to be run with -xmangled to leave the names mangled. You then had to pass --demangle to inferno-collapse-dtrace. This PR will give good results for Rust symbols with the default options.

[codecov]

Codecov Report

Merging #134 into master will increase coverage by 0.29%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   89.52%   89.82%   +0.29%     
==========================================
  Files          10       10              
  Lines        1146     1150       +4     
==========================================
+ Hits         1026     1033       +7     
+ Misses        120      117       -3

Impacted Files	Coverage Δ
src/collapse/perf.rs	92.15% <100%> (ø)	⬆️
src/collapse/dtrace.rs	86.36% <91.66%> (+2.36%)	⬆️

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[jasonrhansen]

I did the same for collapse-perf. When I run perf on my machine, the Rust names are mostly demangled correctly, with the only exception I can see being closures. However, the Rust test profiles from upstream flamegraph have symbols that look like the partially demangled symbols from dtrace or sample. I'm guessing older versions of perf didn't demangle Rust as well?