[red-knot] Consolidate `SymbolBindings`/`SymbolDeclarations` state (#16286)
This updates the `SymbolBindings` and `SymbolDeclarations` types to use
a single smallvec of live bindings/declarations, instead of splitting
that out into separate containers for each field.
I'm seeing an 11-13% `cargo bench` performance improvement with this
locally (for both cold and incremental). I'm interested to see if
Codspeed agrees!
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Co-authored-by: Carl Meyer <carl@astral.sh>