Add demangle option to collapsers

[jasonrhansen]

Fixes #87

This is only useful if you pass --no-demangle to perf script, or -xmangled to dtrace.

Since DTrace doesn't do a great job demangling Rust names, you can get better results by leaving names mangled in profiles, then passing --demangle to inferno-collapse-dtrace. It would probably make sense for ferrous-systems/flamegraph to use DTrace this way.

[jonhoo]

Looks like rustfmt is complaining: https://travis-ci.com/jonhoo/inferno/jobs/212451904

[codecov]

Codecov Report

Merging #132 into master will decrease coverage by 0.39%.
The diff coverage is 70.83%.

@@            Coverage Diff            @@
##           master     #132     +/-   ##
=========================================
- Coverage   90.12%   89.72%   -0.4%     
=========================================
  Files           8        8             
  Lines         962      983     +21     
=========================================
+ Hits          867      882     +15     
- Misses         95      101      +6

Impacted Files	Coverage Δ
src/collapse/dtrace.rs	84% <70%> (-1.72%)	⬇️
src/collapse/perf.rs	92.07% <75%> (-0.35%)	⬇️

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