our constness helpers don't do the same as like-named STL helpers

Submitted by Benoit Jacob

Assigned to Hauke Heibel

This follows up on bug #85 (closed).

In presence of pointer types T*, our constness helpers work on the value type T, whereas STL helpers don't special-case pointer types.

hence:

us: T* -> const T*

stl: T* -> T* const

This applies to remove_const and add_const.

I suggest to rename our helpers to slightly different names to prevent confusion with STL. How about:

add_const -> add_const_on_value_type

remove_const -> remove_const_on_value_type

We should then have add_const and remove_const behave exactly like in the stl.