feat(frontends/lean): display metavariable application arguments in check command

The idea is to "fix" counter-intuitive output like the ones were
produced in the tests check.lean and check2.lean

src/frontends/lean/builtin_cmds.cpp

@@ -218,8 +218,10 @@ environment check_cmd(parser & p) {
     auto tc = mk_type_checker(p.env(), p.mk_ngen(), true);
     expr type = tc->check(e, ls).first;
     auto reg              = p.regular_stream();
-    formatter const & fmt = reg.get_formatter();
+    formatter fmt         = reg.get_formatter();
     options opts          = p.ios().get_options();
+    opts                  = opts.update(get_pp_metavar_args_name(), true);
+    fmt                   = fmt.update_options(opts);
     unsigned indent       = get_pp_indent(opts);
     format r = group(fmt(e) + space() + colon() + nest(indent, line() + fmt(type)));
     reg << mk_pair(r, opts) << endl;

src/frontends/lean/pp_options.cpp

@@ -121,6 +121,7 @@ name const & get_pp_coercions_option_name() { return *g_pp_coercions; }
 name const & get_pp_full_names_option_name() { return *g_pp_full_names; }
 name const & get_pp_universes_option_name() { return *g_pp_universes; }
 name const & get_pp_notation_option_name() { return *g_pp_notation; }
+name const & get_pp_metavar_args_name() { return *g_pp_metavar_args; }
 
 unsigned get_pp_max_depth(options const & opts)     { return opts.get_unsigned(*g_pp_max_depth, LEAN_DEFAULT_PP_MAX_DEPTH); }
 unsigned get_pp_max_steps(options const & opts)     { return opts.get_unsigned(*g_pp_max_steps, LEAN_DEFAULT_PP_MAX_STEPS); }

src/frontends/lean/pp_options.h

@@ -11,6 +11,7 @@ name const & get_pp_coercions_option_name();
 name const & get_pp_full_names_option_name();
 name const & get_pp_universes_option_name();
 name const & get_pp_notation_option_name();
+name const & get_pp_metavar_args_name();
 
 unsigned get_pp_max_depth(options const & opts);
 unsigned get_pp_max_steps(options const & opts);

tests/lean/check.lean.expected.out

@@ -1,4 +1,4 @@
 and.intro : ?a → ?b → ?a ∧ ?b
 or.elim : ?a ∨ ?b → (?a → ?c) → (?b → ?c) → ?c
 eq : ?A → ?A → Prop
-eq.rec : ?C → (Π {a : ?A}, ?a = a → ?C)
+eq.rec : ?C ?a → (Π {a : ?A}, ?a = a → ?C a)

tests/lean/check2.lean.expected.out

@@ -1 +1 @@
-eq.rec_on : ?a = ?a_1 → ?C → ?C
+eq.rec_on : ?a = ?a_1 → ?C ?a → ?C ?a_1