This improves the load time of most brew commands. For an example of
one of the simplest commands this speeds up:
Without Bootsnap:
```
$ hyperfine 'git checkout master; brew help' 'git checkout optimise_requires; brew help'
Benchmark 1: git checkout master; brew help
Time (mean ± σ): 525.0 ms ± 35.8 ms [User: 229.9 ms, System: 113.1 ms]
Range (min … max): 465.3 ms … 576.6 ms 10 runs
Benchmark 2: git checkout optimise_requires; brew help
Time (mean ± σ): 383.3 ms ± 25.1 ms [User: 133.0 ms, System: 72.1 ms]
Range (min … max): 353.0 ms … 443.6 ms 10 runs
Summary
git checkout optimise_requires; brew help ran
1.37 ± 0.13 times faster than git checkout master; brew help
```
With Bootsnap:
```
$ hyperfine 'git checkout master; brew help' 'git checkout optimise_requires; brew help'
Benchmark 1: git checkout master; brew help
Time (mean ± σ): 386.0 ms ± 30.9 ms [User: 130.2 ms, System: 93.8 ms]
Range (min … max): 359.5 ms … 469.3 ms 10 runs
Benchmark 2: git checkout optimise_requires; brew help
Time (mean ± σ): 330.2 ms ± 32.4 ms [User: 93.4 ms, System: 73.0 ms]
Range (min … max): 302.9 ms … 413.9 ms 10 runs
Summary
git checkout optimise_requires; brew help ran
1.17 ± 0.15 times faster than git checkout master; brew help
```
- This was done with `brew typecheck --update --suggest-typed` which
(as of the previous commit) uses Spoom, yet another gem. I thought I'd
see how well it works. There are no Sorbet errors after these changes!
- This also required auto-fixes for Layout/EmptyLinesAroundBlockBody and
Layout/InconsistentIndentation once the auto-fixer had got rid of the
"redundant begin"s.
Replaces our serialization of child process
errors via Marshal with JSON, preventing
unintentional or malicious code execution outside
of the build sandbox.
Additionally, adds tests for the new behavior.
