feat(emacs/lean-server): print out signal event to debug buffer
chore(.travis): install valgrind before we install c++ compiler
chore(frontends/lean/elaborator): minor cleanup
refactor(frontends/lean/elaborator): move local_context to separate file
chore(tests): use option -t 100000 to speedup tests
refactor(frontends/lean/elaborator): rename choice_elaborator to choice_iterator and move to separate module