3.3.1

This is a patch release for multiple small but inconvenient bugs.

• Prevent crash when doing a hash check with missing index, and fetch the index instead
• Atomically load changed erlang modules on OTP >= 19 in rebar3 shell; fix reloading nifs.
• Print stacktrace in a more conventional way in errors
• reset accumulated coverdata across providers on writing out to disk (fixes inaccurate counts)
• Add support for cover_excl_mods to exclude modules from code coverage