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
```
Fixes (on linux), for example for the ninja formula test:
```
pgrep
-P
83252
{out: :close}}
Invalid preceding regular expressionpgrep: write error: Bad file descriptor
Error: ninja: failed
An exception occurred within a child process:
BuildError: Failed executing: pgrep -P 83252 {out: :close}
```
Using File:NULL is more portable.
If people are running `brew test` but don't have `pgrep` or `pkill`
installed the forked processes may never be killed. This is not a big
deal on a user's machine but on CI may make a job block and never
complete/timeout successfully.
This should make it easier to cleanup after test failures or successes.
It may also allow simplification of `test do` blocks if they can assume
that all subprocesses will be killed.
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.
