Improvements in documentation, interface, diagnostic output

New in this release:

• Updated build/test instructions for LLVM flang-new. 🏗️
• Renaming of an internally-used module, function, and macro to reduce the likelihood of name clashes with user code. 🌹
• Comments in the public interface module, assert_m, demarcating the part not intended for explicit reference in user code. 📝
• More controllable diagnostic output so padding of line numbers is the same across compilers. 🔢

What's Changed

• Interface and documentation improvements by @rouson in #52

Full Changelog: 2.0.1...2.1.0