## Summary

This PR fixes the performance degradation introduced in
#6345. Instead of using the
generic `Nodes` structs, we now use separate `Statement` and
`Expression` structs. Importantly, we can avoid tracking a bunch of
state for expressions that we need for parents: we don't need to track
reference-to-ID pointers (we just have no use-case for this -- I'd
actually like to remove this from statements too, but we need it for
branch detection right now), we don't need to track depth, etc.

In my testing, this entirely removes the regression on all-rules, and
gets us down to 2ms slower on the default rules (as a crude hyperfine
benchmark, so this is within margin of error IMO).

No behavioral changes.

## Summary

When we iterate over the AST for analysis, we often process nodes in a
"deferred" manner. For example, if we're analyzing a function, we push
the function body onto a deferred stack, along with a snapshot of the
current semantic model state. Later, when we analyze the body, we
restore the semantic model state from the snapshot. This ensures that we
know the correct scope, hierarchy of statement parents, etc., when we go
to analyze the function body.

Historically, we _haven't_ included the _expression_ hierarchy in the
model snapshot -- so we track the current expression parents in the
visitor, but we never save and restore them when processing deferred
nodes. This can lead to subtle bugs, in that methods like
`expr_parent()` aren't guaranteed to be correct, if you're in a deferred
visitor.

This PR migrates expression tracking to mirror statement tracking
exactly. So we push all expressions onto an `IndexVec`, and include the
current expression on the snapshot. This ensures that `expr_parent()`
and related methods are "always correct" rather than "sometimes
correct".

There's a performance cost here, both at runtime and in terms of memory
consumption (we now store an additional pointer for every expression).
In my hyperfine testing, it's about a 1% performance decrease for
all-rules on CPython (up to 533.8ms, from 528.3ms) and a 4% performance
decrease for default-rules on CPython (up to 212ms, from 204ms).
However... I think this is worth it given the incorrectness of our
current approach. In the future, we may want to reconsider how we do
these upward traversals (e.g., with something like a red-green tree).
(**Note**: in astral-sh#6351, the slowdown
seems to be entirely removed.)

…#6351)

## Summary

This PR fixes the performance degradation introduced in
astral-sh#6345. Instead of using the
generic `Nodes` structs, we now use separate `Statement` and
`Expression` structs. Importantly, we can avoid tracking a bunch of
state for expressions that we need for parents: we don't need to track
reference-to-ID pointers (we just have no use-case for this -- I'd
actually like to remove this from statements too, but we need it for
branch detection right now), we don't need to track depth, etc.

In my testing, this entirely removes the regression on all-rules, and
gets us down to 2ms slower on the default rules (as a crude hyperfine
benchmark, so this is within margin of error IMO).

No behavioral changes.

## Summary

This PR is a follow-up to the suggestion in
#6345 (comment) 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`