Add metaprogram::is_at_start() function
Make Clang binary available for tests on Linux
Show Clang discovery details in CI
Stop using clang_default_path in CI
Enable constexpr testing in CI
Display Clang errors in failing tests
Add CLANG_LIBRARYDIR in CI build
Support finding (non-templight) libclang.so when it is not in /usr/lib
Use 2 cores in CI
Remove unused ifndef
Implement metaprogram::step_back and add step -n

Memory usage is too high for large metaprograms
Remove precondition in command_step
Improve step_back's memory requirements by using rollback objects
Make mdb_shell's metaprogram instance optional
Merge branch 'step-back'