feat(kernel/expr): use the expression depth in the hash code

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

src/kernel/expr.cpp

@@ -131,7 +131,9 @@ expr_app::expr_app(expr const & fn, expr const & arg):
                    fn.has_param_univ() || arg.has_param_univ(),
                    std::max(get_depth(fn), get_depth(arg)) + 1,
                    std::max(get_free_var_range(fn), get_free_var_range(arg))),
-    m_fn(fn), m_arg(arg) {}
+    m_fn(fn), m_arg(arg) {
+    m_hash = ::lean::hash(m_hash, m_depth);
+}
 void expr_app::dealloc(buffer<expr_cell*> & todelete) {
     dec_ref(m_fn, todelete);
     dec_ref(m_arg, todelete);

src/kernel/expr.h

@@ -214,6 +214,7 @@ public:
 
 /** \brief Composite expressions */
 class expr_composite : public expr_cell {
+protected:
     unsigned m_depth;
     unsigned m_free_var_range;
     friend unsigned get_depth(expr const & e);