inconsistent behavior with respect to lifetime elision

[nikomatsakis]

In #518 the behavior with respect to lifetime elision for tracked functions is kind of inconsistent:

• For the &dyn Db parameter, we always infer the elided lifetime to be 'db
• For other random inputs, we basically disallow elision: if you use a '_, it will wind up in the value of an associated type, and give an error.
• We'll also do one weird thing which is if you write fn foo<'db>(db: &dyn Db, x: Foo<'db>), I believe the db becomes &'db dyn Db.

In reality, there is only one lifetime you can correctly use, which is 'db -- salsa tracked functions must not take references except for a case like TrackedStruct<'db> or InternedStruct<'db>.

The current setup seems weird but there are two different choices we could make:

• Introduce a 'db if it's not already there and replace all elided lifetimes with 'db.
• Forbid elided lifetimes in the inputs unless there is exactly one (i.e., on the database). This is ~the current behavior, actually, except for the third bullet, which we should fix.

The former seems more convenient: there's only one thing you could want, so let's do it.

The latter is more consistent with non-tracked-functions, and it means if you remove the #[salsa::tracked], your function keeps compiling.

I'm inclined towards the latter for now.