I switched the `assert!` calls in `RefCell` over to `debug_assert!`.
There are probably other instances that should be converted as well, but
I couldn't think of any off the top of my head.

RFC: 0015-assert

feat: show only missing variant suggestion for enums in patterns completion and bump them in list too

Fixes rust-lang#12438

### Points to help in review:

- This PR can be reviewed commit wise, first commit is about bumping enum variant completions up in the list of completions and second commit is about only showing enum variants which are not complete
- I am calculating missing variants in analysis.rs by firstly locating the enum and then comparing each of it's variant's name and checking if arm string already contains that name, this is kinda hacky but I didn't want to implement complete missing_arms assist here as that would have been too bulky to run on each completion cycle ( if we can improve this somehow would appreciate some inputs on it )

### Output:

https://user-images.githubusercontent.com/49019259/208245540-57d7321b-b275-477e-bef0-b3a1ff8b7040.mov

Relevant Zulip Discussion: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Issue.20.2312438

This addresses rust-lang#13099 for the manual_split_once test, using the
str_splitn lint.

changelog: [str_splitn]: Updated str_splitn to use multipart_suggestions
where appropriate