Handle trait alias definitions
Part of #2773
This PR adds a bunch of structs and enum variants for trait aliases. Trait aliases should be handled as an independent item because they are semantically distinct from traits.
I basically started by adding `TraitAlias{Id, Loc}` to `hir_def::item_tree` and iterated adding necessary stuffs until compiler stopped complaining what's missing. Let me know if there's still anything I need to add.
I'm opening up this PR for early review and stuff. I'm planning to add tests for IDE functionalities in this PR, but not type-related support, for which I put FIXME notes.
Fix associated item visibility in block-local impls
Fixes#14046
When we're resolving visibility of block-local items...
> `self` normally refers to the containing non-block module, and `super` to its parent (etc.). However, visibilities must only refer to a module in the DefMap they're written in, so we restrict them when that happens. ([link])
...unless we're resolving visibility of associated items in block-local impls, because that impl is semantically "hoisted" to the nearest (non-block) module. With this PR, we skip the adjustment for such items.
Since visibility representation of those items is modified, this PR also adjusts visibility rendering in `HirDisplay`.
[link]: a6603fc21d/crates/hir-def/src/nameres/path_resolution.rs (L101-L103)
A Resolver *always* has a module scope at the end of its scope stack,
instead of encoding this as an invariant we can just lift this scope
out into a field, allowing us to skip going through the scope vec
indirection entirely.
