`const MutableProxy` might behave unexpectedly

Currently, `const MutableProxy` does not provide write access to the underlying object and forces the user to use `MutableProxy`. But this widens the contract and allows the callee to modify the proxy to point somewhere else.

To me this seems unexpected as this is contrary to how `const iterator_t` and `type_t * const` behave. They provide write access but the user is not able to redirect the iterator or pointer to a different object.