feat: parsing & hir of impl blocks

[baszalmstra]

This is the first PR to support impl blocks for structs. Features included in this PR are:

• Parsing of impl blocks. (ast::Impl and friends)
• Support impl blocks in ItemTree.
• Resolve self type of impl
• Collect all impls from a package, or for a specific (struct) type.
• Several diagnostics related to impl blocks. (See tests in method_resolution.rs)
• Impl code model to interact with impl blocks

Notable features that are still missing and will be addressed in followup PRs:

• self type.
• Code generation.
• Only functions are support in impls. (type alias is missing)
• Language server support
• Docs

[codecov[bot]]

Codecov Report

Attention: 138 lines in your changes are missing coverage. Please review.

Comparison is base (99e5217) 87.37% compared to head (9290fd5) 87.42%. Report is 3 commits behind head on main.

Files	Patch %	Lines
crates/mun_hir/src/code_model/impl.rs	53.42%	34 Missing :warning:
crates/mun_hir/src/method_resolution.rs	87.50%	27 Missing :warning:
crates/mun_syntax/src/ast/extensions.rs	0.00%	18 Missing :warning:
crates/mun_hir/src/diagnostics.rs	33.33%	14 Missing :warning:
crates/mun_syntax/src/ast/generated.rs	76.66%	14 Missing :warning:
crates/mun_hir/src/has_module.rs	64.00%	9 Missing :warning:
crates/mun_hir/src/ids.rs	60.00%	4 Missing :warning:
crates/mun_syntax/src/parsing/grammar/traits.rs	82.60%	4 Missing :warning:
crates/mun_hir/src/utils.rs	57.14%	3 Missing :warning:
..._diagnostics/src/hir/duplicate_definition_error.rs	88.23%	2 Missing :warning:
... and 8 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #551 +/- ## ========================================== + Coverage 87.37% 87.42% +0.05% ========================================== Files 271 276 +5 Lines 31309 31970 +661 ========================================== + Hits 27357 27951 +594 - Misses 3952 4019 +67 ```

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