* feat: lazy compile

* refactor: lazy -> only_changed

* refactor: leave the changed function up to the user

* refactor: pass a function, based on @YannickJadoul and @HDembinski's suggestions

* refactor: old -> _old, as it's not intended for users

* docs: slight improvmenent from @rwgk

* docs: Ccache spelling, extra warning about pip caching

Ccache spelling noted by @YannickJadoul