Speed up new backtracking parser

[isidentical]

The current revision of the new backtracking parser (that gets enabled with -t py310) is kind of slow.

For regular inputs that only use match/case without using the pattern matching statements, it is somewhere between 3 to 4 X slower at the moment (going up to 6 or 7 on hand crafted inputs). On some inputs (like Lib/test/test_patma.py) that contain match/case hundreds of times, with new PEP 634 statements, it takes multiple minutes (2 minutes 34 seconds to be exact) to format these.

I don't know if there is any definite way to speed-up these, but I have an idea which I'll post shortly as a draft PR.