Rename `Option`, add `Assert` trait

Another proposal inspired by an idea from @aturon. Goes well with #243, but isn't tied to it.

[CLICKME](https://github.com/glaebhoerl/rfcs/blob/rename-option-add-assert-trait/active/0000-rename-option-add-assert-trait.md)
The `file!()`, `line!()` calls here are pretty useless, aren't they?