v0.5.6 release

fixes a weird old oversight where �ack() required &mut self for no reason.

0.5.5 release

* fmts/clippy's all the code
* fixes all the derelict CI (and adds miri CI for all the tests)
* adds Entry::or_default and Entry::and_modify
* adds LinkedHashMap::drain

this is still basically in maintenance mode but I used this in anger enough recently to want to clean it up.