Add Type::lifetimes to make getting lifetimes out of Type easier

[QnnOkabayashi]

Forgive me because it's been a long time since I've looked at this repo- I don't remember how everything works but I wanted to get back into it.

I was looking at #406 and saw this part:

There is a way to get an exhaustive Lifetimes for every struct field: basically something like LifetimeEnv.new_lifetimes_for(&field), producing a list of relevant indices. This list is transitively evaluated.

I thought a good first step for this would be to make getting the lifetimes of a field easy, so I added Type::lifetimes which returns a &[MaybeStatic<TypeLifetime>], essentially just consolidating where lifetimes are stored in the Type enum. This will allow for a higher-level api to use this and SubtypeLifetimeVisitor to achieve what I quoted above with something like this:

let mut lifetimes = vec![];
let mut visitor = SubtypeLifetimeVisitor::new(&struct_.lifetime_env, |lt| lifetimes.push(lt));
for field in &struct_.fields {
    for lifetime in field.ty.lifetimes() { // <-- added this method!
        if let MaybeStatic::NonStatic(lt) = lifetime {
            visitor.visit_subtypes(lt);
        }
    }
}

Today this doesn't compile because MethodLifetime != TypeLifetime, but SubtypeLifetimeVisitor would probably be adjusted once each type declaration has their own LifetimeEnv.

[Manishearth]

My plan is actually to have Field resolve lifetimes, but this helps attain that.