perf: add `replaceImpl`

It uses the kernel implementation.
We will replace `Expr.replace` with it after update-stage0

src/Lean/Util/ReplaceExpr.lean

diff --git a/src/Lean/Util/ReplaceExpr.lean b/src/Lean/Util/ReplaceExpr.lean
index d7a364e..deca88c 100644
--- a/src/Lean/Util/ReplaceExpr.lean
+++ b/src/Lean/Util/ReplaceExpr.lean
@@ -77,6 +77,10 @@
     | .proj _ _ b      => let b := replaceNoCache f? b; e.updateProj! b
     | e                => e
 
+
+@[extern "lean_replace_expr"]
+opaque replaceImpl (f? : @& (Expr → Option Expr)) (e : @& Expr) : Expr
+
 @[implemented_by ReplaceImpl.replaceUnsafe]
-partial def replace (f? : Expr → Option Expr) (e : Expr) : Expr :=
+def replace (f? : Expr → Option Expr) (e : Expr) : Expr :=
   e.replaceNoCache f?

src/kernel/replace_fn.cpp

diff --git a/src/kernel/replace_fn.cpp b/src/kernel/replace_fn.cpp
index 3c37d90..23883f1 100644
--- a/src/kernel/replace_fn.cpp
+++ b/src/kernel/replace_fn.cpp
@@ -81,4 +81,73 @@
 expr replace(expr const & e, std::function<optional<expr>(expr const &, unsigned)> const & f, bool use_cache) {
     return replace_rec_fn(f, use_cache)(e);
 }
+
+class replace_fn {
+    std::unordered_map<lean_object *, expr> m_cache;
+    lean_object * m_f;
+
+    expr save_result(expr const & e, expr const & r, bool shared) {
+        if (shared)
+            m_cache.insert(mk_pair(e.raw(), r));
+        return r;
+    }
+
+    expr apply(expr const & e) {
+        bool shared = false;
+        if (is_shared(e)) {
+            auto it = m_cache.find(e.raw());
+            if (it != m_cache.end())
+                return it->second;
+            shared = true;
+        }
+
+        lean_inc(e.raw());
+        lean_inc_ref(m_f);
+        lean_object * r = lean_apply_1(m_f, e.raw());
+        if (!lean_is_scalar(r)) {
+            expr e_new(lean_ctor_get(r, 0));
+            lean_dec_ref(r);
+            return save_result(e, e_new, shared);
+        }
+
+        switch (e.kind()) {
+        case expr_kind::Const: case expr_kind::Sort:
+        case expr_kind::BVar:  case expr_kind::Lit:
+        case expr_kind::MVar:  case expr_kind::FVar:
+            return save_result(e, e, shared);
+        case expr_kind::MData: {
+            expr new_e = apply(mdata_expr(e));
+            return save_result(e, update_mdata(e, new_e), shared);
+        }
+        case expr_kind::Proj: {
+            expr new_e = apply(proj_expr(e));
+            return save_result(e, update_proj(e, new_e), shared);
+        }
+        case expr_kind::App: {
+            expr new_f = apply(app_fn(e));
+            expr new_a = apply(app_arg(e));
+            return save_result(e, update_app(e, new_f, new_a), shared);
+        }
+        case expr_kind::Pi: case expr_kind::Lambda: {
+            expr new_d = apply(binding_domain(e));
+            expr new_b = apply(binding_body(e));
+            return save_result(e, update_binding(e, new_d, new_b), shared);
+        }
+        case expr_kind::Let: {
+            expr new_t = apply(let_type(e));
+            expr new_v = apply(let_value(e));
+            expr new_b = apply(let_body(e));
+            return save_result(e, update_let(e, new_t, new_v, new_b), shared);
+        }}
+        lean_unreachable();
+    }
+public:
+    replace_fn(lean_object * f):m_f(f) {}
+    expr operator()(expr const & e) { return apply(e); }
+};
+
+extern "C" LEAN_EXPORT obj_res lean_replace_expr(b_obj_arg f, b_obj_arg e) {
+    expr r = replace_fn(f)(TO_REF(expr, e));
+    return r.steal();
+}
 }