feat(emacs): use lean-mode for .hlean
feat(*): add support for separate HoTT library
feat(build): add HoTT library to build
fix(build): clean-olean target should also clean HoTT library
feat(hott/init): add initialization files
fix(frontends/lean/parser): generate error when 'exit' command is used

m_theorem_queue.join() method assumes there are no open namespaces/scopes
fix(shell): use --server for .hlean files
fix(ltags): support .hlean files