Make std::{set,map} has() and has_key() methods const

They don't modify the object, so should be callable on const objects.

Lib/c/std_map.i

@@ -52,8 +52,8 @@ namespace std {
           else
             throw std::out_of_range("key not found");
         }
-        bool has_key(const K& key) {
-          std::map< K, T, C >::iterator i = self->find(key);
+        bool has_key(const K& key) const {
+          std::map< K, T, C >::const_iterator i = self->find(key);
           return i != self->end();
         }
       }

Lib/c/std_set.i

@@ -40,7 +40,7 @@ namespace std {
         bool del(const T& item) {
           return self->erase(item) != 0;
         }
-        bool has(const T& item) {
+        bool has(const T& item) const {
           return self->count(item) != 0;
         }
       }