Add support for collapsing PHP Xdebug traces

[MaartenStaa]

Inspired by and continued from #126 by @daniellockyer

This PR is a bit different, because in the meantime Xdebug 3 was released, which apparently introduced a new tracing format as far as I can tell based on the samples I've taken, and the old code. One caveat is that Xdebug actually produces gzipped files, which is not (yet) handled here, as it seems a bit out of scope for this tool.

I went through the comments on the old PR and think I've tackled them all, but please let me know if you have any other feedback.

Also, I love your streams and your book, keep up the great work :)

[codecov]

Codecov Report

Merging #253 (0dccda7) into master (acc8981) will decrease coverage by 0.50%.
The diff coverage is 79.37%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
- Coverage   87.32%   86.82%   -0.51%     
==========================================
  Files          18       19       +1     
  Lines        2525     2687     +162     
==========================================
+ Hits         2205     2333     +128     
- Misses        320      354      +34     

Impacted Files	Coverage Δ
src/collapse/mod.rs	52.63% <ø> (ø)
src/collapse/xdebug.rs	78.98% <78.98%> (ø)
src/collapse/guess.rs	72.09% <100.00%> (+1.36%)	⬆️
src/flamegraph/svg.rs	75.90% <0.00%> (-0.18%)	⬇️

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