Infer implied outlives from fn signature

[kirloo]

What does this PR do?

Closes #405
Closes #202

How does it work, what questions do you have?

I created a new function implied_outlives_for_fn which goes through input argument and output types for a function and produces a Wcs corresponding to the function's implied outlives bounds. When encountering a ref it creates a WhereClause::Outlives for the inner type and the outer lifetime. The function recurses to handle nested borrows. The final Wcs is included in the assumptions of check_fn.

I think the logic seems sound and all tests pass, but:

• I'm not sure if I'm handling the case of multiple type parameters correctly (e.g. &'a HashMap<&'b K,&'c V>)
• I'm not sure if I'm handling the Arc in Parameter::Lt correctly.
• I'm not sure about function names and file structure.

Some old tests had to be updated due to the new assumptions.

This is a second attempt at #407 after I messed up the history trying to merge test updates.

AI disclosure

• I used an AI tool for research, autocomplete, or in other minimal ways

[rustbot]

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[kirloo]

@nikomatsakis Now everything is in judgment functions

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.