feat(emacs/lean-option): show the current value of an option

close #125
refactor(library): add type_util module, and move get_expect_num_args to it

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/server): add FINDG command (find declarations that can be used to fill a placeholder)

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