Introduce IR frontend

[AlexDenisov]

This PR introduces an IR Frontend: a shared library that can be directly integrated into the build system.
Example:

> clang++ -fexperimental-new-pass-manager -fpass-plugin=/usr/lib/mull-ir-frontend-12 main.cpp
> mull-runner-12 a.out # a.out has all the mutations

Notes:

• it doesn't work with Clang 8 (and earlier)
• Clang 9/10/11 requires optimization level to be not -O0 (e.g. -O1, -O2). Any newer version of Clang work without optimizations as well
• with the plugin in place we can deprecate and remove mull-cxx, thus simplifying Mull
• most of the tests were extended to replicate mull-cxx behaviour

TODO:

• rewrite Hacking on Mull doc
• rewrite the tutorials to use the new approach
• add a deprecation notice to the mull-cxx
• add docs for the configuration