fix(frontends/lean): unprotected call to Lua API

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs): useful abbreviations

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs): highlight tactics

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean): When and Cond tactical tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs): more highlighting

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