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)
Yeah. But you were never going to get an informative message here anyways (and `assert_msg` expects them). For comparison, if I had directly used `fail!()` here, it would have the same effect w.r.t. file/line numbers.