delegation: refactor AST -> HIR lowering

[aerooneqq]

After a lot of new features were added to delegation the code became a bit worse, as resolution and lowering logic is mixed in delegation.rs. In this file the output of resolution step of delegation is transferred as function params (in some functions up to 7 or 8 params), which is not good looking for me.

As I mentioned in the previous paragraph delegation AST -> HIR lowering can be split into two phases:

1. Resolution - we collect all information about signature, signature parent context, delegation parent, delegation generics. Then this information is used in AST -> HIR lowering,
2. AST -> HIR lowering - we generate everything based on the output of resolution.

This PR explicitly splits those two phases: as a first step we collect all needed information into DelegationResolution struct and then use it during AST -> HIR lowering. The delegation.rs file is moved into delegation folder and renamed to mod.rs, the final file structure is as follows:

• mod.rs - contains lowering logic for everything but generics,
• generics.rs - contains logic for resolution and uplifting of generic params, placed in a separate file as it is a significant and independent part of lowering,
• attributes.rs - contains relatively small piece of logic which connects to attributes inheritance,
• resolution.rs - contains logic for resolution of everything but generics.

As resolution of delegation and generics logically does not depend on LoweringContext, as from its name LoweringContext contains many additional information which is used for lowering, I've created LoweringContextForResolution trait which abstracts operations which are used during delegation resolution.

Benefits of this PR:

• Resolution information is grouped into single struct, is extracted from AST -> HIR lowering which leads to better transfer of resolution across lowering (through a single struct, not through many arguments) and enhanced understanding of which information we use for delegation lowering,
• There is now a single path for generating error delegation - if an error was returned from delegation resolution,
• Refactorings of uplift_delegation_generics which was creating generic params for delegation made the code more understandable.

Part of #118212.
r? @petrochenkov

[aerooneqq]

Now this PR is based on #158397, so it is draft until #158397 is merged, but feedback is anyway welcomed.

[rust-bors]

☔ The latest upstream changes (presumably #158455) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.