feat(kernel/level): improve universe level pretty printer

Example: produce `l+2` instead of `succ (succ l)`.

src/kernel/level.cpp

@@ -517,8 +517,10 @@ format pp(level l, bool unicode, unsigned indent) {
             return format(to_param_core(l).m_id);
         case level_kind::Meta:
             return format("?") + format(meta_id(l));
-        case level_kind::Succ:
-            return group(compose(format("succ"), nest(indent, compose(line(), pp_child(succ_of(l), unicode, indent)))));
+        case level_kind::Succ: {
+            auto p = to_offset(l);
+            return format{pp_child(p.first, unicode, indent), format("+"), format(p.second)};
+        }
         case level_kind::Max: case level_kind::IMax: {
             format r = format(is_max(l) ? "max" : "imax");
             r += nest(indent, compose(line(), pp_child(to_max_core(l).m_lhs, unicode, indent)));

tests/lean/prodtst.lean.expected.out

@@ -1 +1 @@
-prod.{l_1 l_2} : Type.{succ l_1} → Type.{succ l_2} → Type.{max (succ l_1) (succ l_2)}
+prod.{l_1 l_2} : Type.{l_1+1} → Type.{l_2+1} → Type.{max (l_1+1) (l_2+1)}

tests/lean/univ_vars.lean.expected.out

@@ -2,4 +2,4 @@ id1 : Π (A : Type.{u}), A → A
 id2.{l_2} : Π (B : Type.{l_2}), B → B
 id3.{l_2} : Π (C : Type.{l_2}), C → C
 foo.{l_2} : Π (A₁ A₂ : Type.{l_2}), A₁ → A₂ → Prop
-Type.{m} : Type.{succ m}
+Type.{m} : Type.{m+1}