Add an `insert` method to `KeyedContainer`

Mentioned by @graven at Rec!4423:

in order to avoid mistakes like this, we should add an insert method to KeyedContainer which accepts a std::unique_ptr<T> as argument, and deprecate the method that takes a plain pointer. That way, one is forced to write:


     output.insert( std::move(outp) );


instead, at which point there is no reason to call release() on an std::unique_ptr when inserting into the container, which then makes the remaining uses of release() more suspect, and thus easier/faster/more likely to spot mistakes like this.