C++: const_iterator not comparable to iterator for map<>s

[Benjamin Kosnik]

> No problem (I hope), I'll try to do that.
> Is a modification of the test 23_containers/map_operators.cc 
> acceptable ? Or should I create a different test ?

I suspect we'll need two files. We need both positive (should compile) 
and negative (should not compile) tests.

-benjamin