Add SnoopPrecompile

[jakobnissen]

The small dependency SnoopPrecompile helps precompilation, while itself having
little impact on latency.
With this change, JSON3 precompiles significantly better.

This change increases time spent on using, but decreases inference time for any of the workloads in the workload.jl file.

Here are timings for a script which loads JSON3 and executs the same workload in workload.jl file on master vs this PR:

# Master
Benchmark 1: julia --project=. --startup=no /tmp/foo.jl
  Time (mean ± σ):      3.531 s ±  0.065 s    [User: 3.537 s, System: 0.580 s]
  Range (min … max):    3.453 s …  3.697 s    10 runs

# PR
Benchmark 1: julia --project=. --startup=no /tmp/foo.jl
  Time (mean ± σ):      2.149 s ±  0.031 s    [User: 2.195 s, System: 0.554 s]
  Range (min … max):    2.099 s …  2.206 s    10 runs

@quinnj I'm not sure the workload covers all the common use-cases of JSON3, i.e. whether the workload ends up calling all the methods a user would typically call.

[jakobnissen]

The workload from #210 is cut from 1.94s to 1.20s after this change.

[jakobnissen]

@quinnj is it possible to merge this? The precise SnoopPrecompile statements can easily be updated later.

[codecov]

Codecov Report

Base: 88.36% // Head: 88.30% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (bd1b5a2) compared to base (3d542d3).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
- Coverage   88.36%   88.30%   -0.06%     
==========================================
  Files           9        9              
  Lines        1710     1710              
==========================================
- Hits         1511     1510       -1     
- Misses        199      200       +1     

Impacted Files	Coverage Δ
src/JSON3.jl	100.00% <ø> (ø)
src/strings.jl	98.91% <0.00%> (-1.09%)	⬇️

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