gh-153568: Reuse shared prefixes in generated parser rules

[pablogsal]

For obj[index], the parser tries the attribute and call alternatives before the subscript one. Each choice starts by asking for the same obj again. The parse cache saves us from parsing it from scratch, but we still pay for another rule call and cache lookup.

This teaches the generator to keep that result and the position where it ended in local variables. When one choice fails, the next one can pick them up directly and try its own ending. It does this for adjacent choices that start with the same memoized rule, including expression, arithmetic and assignment-target rules.

The grammar stays as it is. The generator finds these shared starts on its own.

On my machine, this makes parsing the standard library and twenty repositories about 10.7% faster overall.

Codebase	Python files	Parsing speed
Standard library	2,054	11.5% faster
Django	2,932	11.7% faster
pandas	1,527	11.6% faster
NumPy	495	11.7% faster
SciPy	999	10.7% faster
scikit-learn	1,036	10.0% faster
SymPy	1,607	11.6% faster
Transformers	4,884	10.7% faster
PyTorch	4,890	10.5% faster
Home Assistant	18,731	11.1% faster
mypy	442	9.7% faster
Requests	37	11.3% faster
Flask	83	11.3% faster
FastAPI	1,138	10.6% faster
SQLAlchemy	673	11.3% faster
Black	345	9.4% faster
pytest	274	10.7% faster
Ansible	1,853	10.6% faster
Scrapy	491	11.9% faster
Celery	428	10.2% faster
Sphinx	774	10.6% faster

The table compares this change against main at fd0970c0ab, using release builds without PGO or LTO. Each result is the median of five paired runs. The timings include tokenization, building the AST and cleanup, without compiling bytecode or running the files.

This overlaps with #157437, which shares some of these starts by rewriting the grammar. Adding this on top of that PR still gives about 5.2% faster parsing across the standard library, mypy, Home Assistant and PyTorch in a separate run. That comparison does not include the rest of the pending parser changes.

Part of #153568.

• Issue: Speed up the parser #153568