feat(bin/lmake): add --jobs and --keep-going options
fix(emacs): remove lean-get-this-if-true-or-that
feat(emacs/lean-flycheck): customize checker name and options
feat(emacs): implement lean-show-type
refactor(frontends/lean): rename elaborator_env to elaborator_context

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>