Add coverage support for Ruby code, not just C extensions

[mschwager]

• https://github.com/google/atheris/blob/2.3.0/src/native/tracer.cc
• https://llvm.org/docs/LibFuzzer.html#tracing-cmp-instructions
• https://llvm.org/docs/LibFuzzer.html#value-profile
• https://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow
• https://www.code-intelligence.com/blog/java-fuzzing-with-jazzer

[mschwager]

Might be able to use/hook Ruby's internal coverage functionality:

• https://docs.ruby-lang.org/en/3.3/TracePoint.html
• https://docs.ruby-lang.org/en/3.3/Coverage.html
• https://github.com/ruby/ruby/blob/v3_3_0/ext/coverage/coverage.c
• https://github.com/ruby/ruby/blob/v3_3_0/vm_core.h#L2183-L2184
• https://github.com/ruby/ruby/blob/v3_3_0/iseq.c
• https://github.com/ruby/ruby/blob/v3_3_0/thread.c

[mschwager]

• https://github.com/golang/go/issues/14565
• https://cyruscyliu.github.io/posts/2021-11-02-libFuzzer-cov-control/