Use a single node hierarchy to track statements and expressions (#6709)

## Summary

This PR is a follow-up to the suggestion in
https://github.com/astral-sh/ruff/pull/6345#discussion_r1285470953 to
use a single stack to store all statements and expressions, rather than
using separate vectors for each, which gives us something closer to a
full-fidelity chain. (We can then generalize this concept to include all
other AST nodes too.)

This is in part made possible by the removal of the hash map from
`&Stmt` to `StatementId` (#6694), which makes it much cheaper to store
these using a single interface (since doing so no longer introduces the
requirement that we hash all expressions).

I'll follow-up with some profiling, but a few notes on how the data
requirements have changed:

- We now store a `BranchId` for every expression, not just every
statement, so that's an extra `u32`.
- We now store a single `NodeId` on every snapshot, rather than separate
`StatementId` and `ExpressionId` IDs, so that's one fewer `u32` for each
snapshot.
- We're probably doing a few more lookups in general, since any calls to
`current_statement()` etc. now have to iterate up the node hierarchy
until they identify the first statement.

## Test Plan

`cargo test`

crates/ruff_python_semantic/src/lib.rs

@@ -3,22 +3,20 @@ mod binding;
 mod branches;
 mod context;
 mod definition;
-mod expressions;
 mod globals;
 mod model;
+mod nodes;
 mod reference;
 mod scope;
 mod star_import;
-mod statements;
 
 pub use binding::*;
 pub use branches::*;
 pub use context::*;
 pub use definition::*;
-pub use expressions::*;
 pub use globals::*;
 pub use model::*;
+pub use nodes::*;
 pub use reference::*;
 pub use scope::*;
 pub use star_import::*;
-pub use statements::*;