Support generic function in generate_function assist

[lowr]

Part of #3639

This PR adds support for generic function generation in generate_function assist. Now the assist looks for generic parameters and trait bounds in scope, filters out irrelevant ones, and generates new function with them.

See fn_generic_params() for the outline of the procedure, and see comments on filter_unnecessary_bounds() for criteria for filtering. I think it's good criteria for most cases, but I'm open to opinions and suggestions.

The diff is pretty big, but it should run in linear time w.r.t. the number of nodes we operate on and should be fast enough.

Some notes:

• When we generate function in an existing impl, generic parameters may cause name conflict. While we can detect the conflict and rename conflicting params, I didn't find it worthwhile mainly because it's really easy to resolve on IDE: use Rename functionality.
• I've implemented graph structure myself, because we don't have graph library as a dependency and we only need the simplest one.
  • Although petgraph is in our dependency graph and I was initially looking to use it, we don't actually depend on it AFAICT since it's only used in chalk's specialization graph handling, which we don't use. I'd be happy to replace my implementation with petgraph if it's okay to use it though.
• There are some caveats that I consider out of scope of this PR. See FIXME notes on added tests.

[lowr]

I'm not sure where to place this function. hir_ty::utils says it's for "Helper functions for working with def, which (...) can't be computed directly from *Data" and it doesn't seem to fit there?

[Veykril]

Not using petgraph here seems okay since this doesn't seem to need a lot of graph-like functionaly

[Veykril]

r=me with the 2 unwraps replaced
@bors delegate+

[bors]

✌️ @lowr can now approve this pull request

[lowr]

Two rather random thoughts:

• extract_function assist may benefit from functions this PR adds
• I feel it might make sense to add a variant of the assist: "Generate function/method without trait bounds"

@bors r=Veykril

[bors]

📌 Commit 493cabb has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 493cabb with merge eeceba7...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing eeceba7 to master...