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/binding.rs

@@ -11,8 +11,8 @@ use ruff_text_size::TextRange;
 
 use crate::context::ExecutionContext;
 use crate::model::SemanticModel;
+use crate::nodes::NodeId;
 use crate::reference::ResolvedReferenceId;
-use crate::statements::StatementId;
 use crate::ScopeId;
 
 #[derive(Debug, Clone)]
@@ -24,7 +24,7 @@ pub struct Binding<'a> {
     /// The context in which the [`Binding`] was created.
     pub context: ExecutionContext,
     /// The statement in which the [`Binding`] was defined.
-    pub source: Option<StatementId>,
+    pub source: Option<NodeId>,
     /// The references to the [`Binding`].
     pub references: Vec<ResolvedReferenceId>,
     /// The exceptions that were handled when the [`Binding`] was defined.
@@ -185,7 +185,7 @@ impl<'a> Binding<'a> {
     /// Returns the range of the binding's parent.
     pub fn parent_range(&self, semantic: &SemanticModel) -> Option<TextRange> {
         self.source
-            .map(|statement_id| semantic.statement(statement_id))
+            .map(|id| semantic.statement(id))
             .and_then(|parent| {
                 if parent.is_import_from_stmt() {
                     Some(parent.range())