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>
feat(frontends/lean): improve COERCION info

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(doc/server): update server documentation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(emacs/lean-server): clean up debugging message
feat(emacs/lean-info): improve coercion display

close #124
fix(emacs/lean-info): print stale when info is available

fix #127
checkpoint for imenu
checkpoint