[red-knot] Don't use separate ID types for each alist (#16415)

Regardless of whether #16408 and #16311 pan out, this part is worth
pulling out as a separate PR.

Before, you had to define a new `IndexVec` index type for each type of
association list you wanted to create. Now there's a single index type
that's internal to the alist implementation, and you use `List<K, V>` to
store a handle to a particular list.

This also adds some property tests for the alist implementation.

crates/ruff_index/src/lib.rs

@@ -4,7 +4,6 @@
 //! Inspired by [rustc_index](https://github.com/rust-lang/rust/blob/master/compiler/rustc_index/src/lib.rs).
 
 mod idx;
-pub mod list;
 mod slice;
 mod vec;