Update Code Coverage Tools & Documentation

[dmacattack]

• remove coverage script that relies on nightly builds
• rename script using tarpaulin coverage
• add coverage script using llvm-cov
• update ignore
• add readme for tools

example report - extract and open html/index.html up-streamer-rust-llvm-coverage-example.zip

[dmacattack]

Hey @dmacattack -- left a request, could you take a look?

@PLeVasseur completed

[dmacattack]

[#XX] PR name

@PLeVasseur done & done