…orithm. (pythonGH-15558)The purpose of the `unicodedata.is_normalized` function is to answerthe question `str == unicodedata.normalized(form, str)` moreefficiently than writing just that, by using the "quick check"optimization described in the Unicode standard in UAXpythonGH-15.However, it turns out the code doesn't implement the full algorithmfrom the standard, and as a result we often miss the optimization andend up having to compute the whole normalized string after all.Implement the standard's algorithm.  This greatly speeds up`unicodedata.is_normalized` in many cases where our partial variantof quick-check had been returning MAYBE and the standard algorithmreturns NO.At a quick test on my desktop, the existing code takes about 4.4 ms/MB(so 4.4 ns per byte) when the partial quick-check returns MAYBE and ithas to do the slow normalize-and-compare:  $ build.base/python -m timeit -s 'import unicodedata; s = "\uf900"*500000' \      -- 'unicodedata.is_normalized("NFD", s)'  50 loops, best of 5: 4.39 msec per loopWith this patch, it gets the answer instantly (58 ns) on the same 1 MBstring:  $ build.dev/python -m timeit -s 'import unicodedata; s = "\uf900"*500000' \      -- 'unicodedata.is_normalized("NFD", s)'5000000 loops, best of 5: 58.2 nsec per loopThis restores a small optimization that the original version of thiscode had for the `unicodedata.normalize` use case.With this, that case is actually faster than in master!$ build.base/python -m timeit -s 'import unicodedata; s = "\u0338"*500000' \    -- 'unicodedata.normalize("NFD", s)'500 loops, best of 5: 561 usec per loop$ build.dev/python -m timeit -s 'import unicodedata; s = "\u0338"*500000' \    -- 'unicodedata.normalize("NFD", s)'500 loops, best of 5: 512 usec per loop(cherry picked from commit2f09413)Co-authored-by: Greg Price <gnprice@gmail.com>