Add predefined function del/1 on dict with some minimal tests

[esad]

This may need more work, but I just wanted to post this PR for discussion and see if this is something others may find useful. While working with dictionary based data structures, I've come to rely on put(KeyPath, Value), but there is no "function" for deleting keys. This would add another predefined del consistent in naming with del_dict/4 that also accepts a keypath.

[JanWielemaker]

Thanks. Makes sense. We could consider a del(Key,Value), where Value is unified with the current value. The code could be simplified using SSU (=>). And of course, we need documentation ...