Add yams-fuzzer target & run on CI

[jpsim]

Swift can integrate with libFuzzer and seems to work quite well.

To seed the fuzzer with a useful corpus of input data, I'm using the yaml from yaml-test-suite.

References:

• https://github.com/apple/swift/blob/main/docs/libFuzzerIntegration.md
• https://llvm.org/docs/LibFuzzer.html#options

[codecov-commenter]

Codecov Report

Merging #334 (5b55ed9) into main (a207bf7) will decrease coverage by 0.48%. The diff coverage is 60.86%.

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
- Coverage   87.72%   87.24%   -0.49%     
==========================================
  Files          15       15              
  Lines        1418     1435      +17     
==========================================
+ Hits         1244     1252       +8     
- Misses        174      183       +9     

Impacted Files	Coverage Δ
Sources/Yams/YamlError.swift	71.42% <60.00%> (-1.18%)	:arrow_down:
Sources/Yams/Constructor.swift	93.30% <61.11%> (-3.13%)	:arrow_down:

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