feature: refactored RawItems into ItemTree

[baszalmstra]

First refactor to remove RawItems and create better separation of concerns. ItemTree is a derived AST that only contains module items. All interning is now done as ItemTree ids instead of building directly on the AST. Hopefully, this creates a clearer separation between items and the AST.

When adding modules (which Im also working on) having this ItemTree is much clearer than using either RawItems or the code model or a strange combination. Im not sure yet how to handle the TypeRefSourceMap, Im looking into removing that too.

[codecov]

Codecov Report

Merging #271 into master will decrease coverage by 0.00%.
The diff coverage is 91.91%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   79.05%   79.05%   -0.01%     
==========================================
  Files         214      217       +3     
  Lines       12808    12941     +133     
==========================================
+ Hits        10125    10230     +105     
- Misses       2683     2711      +28     

Impacted Files	Coverage Δ
crates/mun_hir/src/source_id.rs	64.28% <ø> (-21.43%)	⬇️
crates/mun_hir/src/item_tree.rs	74.50% <74.50%> (ø)
crates/mun_hir/src/code_model/src.rs	42.30% <78.57%> (+21.25%)	⬆️
crates/mun_hir/src/code_model.rs	81.41% <89.47%> (+1.68%)	⬆️
crates/mun_hir/src/adt.rs	94.73% <100.00%> (+4.73%)	⬆️
crates/mun_hir/src/db.rs	100.00% <100.00%> (ø)
crates/mun_hir/src/expr/validator/tests.rs	100.00% <100.00%> (ø)
crates/mun_hir/src/ids.rs	100.00% <100.00%> (ø)
crates/mun_hir/src/item_tree/lower.rs	100.00% <100.00%> (ø)
crates/mun_hir/src/item_tree/tests.rs	100.00% <100.00%> (ø)
... and 12 more

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[Wodann]

Nice refactor. I don't have anything to improve, except a typo in the comments.