Transition to salsa coarse-grained tracked structs (#15763)

## Summary

Transition to using coarse-grained tracked structs (depends on
https://github.com/salsa-rs/salsa/pull/657). For now, this PR doesn't
add any `#[tracked]` fields, meaning that any changes cause the entire
struct to be invalidated. It also changes `AstNodeRef` to be
compared/hashed by pointer address, instead of performing a deep AST
comparison.

## Test Plan

This yields a 10-15% improvement on my machine (though weirdly some runs
were 5-10% without being flagged as inconsistent by criterion, is there
some non-determinism involved?). It's possible that some of this is
unrelated, I'll try applying the patch to the current salsa version to
make sure.

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_index/Cargo.toml

@@ -15,6 +15,7 @@ doctest = false
 
 [dependencies]
 ruff_macros = { workspace = true }
+salsa = { workspace = true, optional = true }
 
 [dev-dependencies]
 static_assertions = { workspace = true }

crates/ruff_index/src/vec.rs

@@ -181,3 +181,16 @@ impl<I: Idx, T, const N: usize> From<[T; N]> for IndexVec<I, T> {
 // not the phantom data.
 #[allow(unsafe_code)]
 unsafe impl<I: Idx, T> Send for IndexVec<I, T> where T: Send {}
+
+#[allow(unsafe_code)]
+#[cfg(feature = "salsa")]
+unsafe impl<I, T> salsa::Update for IndexVec<I, T>
+where
+    T: salsa::Update,
+{
+    #[allow(unsafe_code)]
+    unsafe fn maybe_update(old_pointer: *mut Self, new_value: Self) -> bool {
+        let old_vec: &mut IndexVec<I, T> = unsafe { &mut *old_pointer };
+        salsa::Update::maybe_update(&mut old_vec.raw, new_value.raw)
+    }
+}