enum CompInterType: make a real enum

[folkertdev]

this PR looks big but most lines are because I needed to reverse two branches so that

if comp_type.is_none() { ... } else { ... }

can turn into

if let Some(comp_type) = comp_type { ... }  else { ... }

the first commit is most changes, the second one just the switching of branches and using the variable bound by the if-let

[kkysen]

Still reviewing the second commit.

[kkysen]

The 2nd commit is too annoying to review. It's just the if let Some and swapping the branches' order, right? If that's it, it LGTM.

[folkertdev]

yes, that and it removes these lines

// TODO use if let in the surrounding if structure
let comp_inter_type = b.c2rust_unnamed.c2rust_unnamed_0.comp_type.unwrap();

because that variable is now bound by the if let