gh-125196: Add PyUnicodeWriter_Fill() function #168

Job	Run time
Interpreter (Debug)	7m 0s
Free-Threaded (Debug)	10m 33s
i686-pc-windows-msvc/msvc (Debug)	17m 50s
i686-pc-windows-msvc/msvc (Release)	21m 0s
x86_64-pc-windows-msvc/msvc (Debug)	17m 28s
x86_64-pc-windows-msvc/msvc (Release)	15m 51s
aarch64-pc-windows-msvc/msvc (Debug)	4m 58s
aarch64-pc-windows-msvc/msvc (Release)	6m 7s
x86_64-apple-darwin/clang (Debug)	14m 22s
x86_64-apple-darwin/clang (Release)	23m 51s
aarch64-apple-darwin/clang (Debug)	8m 45s
aarch64-apple-darwin/clang (Release)	13m 46s
x86_64-unknown-linux-gnu/gcc (Debug)	7m 39s
x86_64-unknown-linux-gnu/gcc (Release)	17m 37s
x86_64-unknown-linux-gnu/clang (Debug)	7m 50s
x86_64-unknown-linux-gnu/clang (Release)	14m 4s
aarch64-unknown-linux-gnu/gcc (Debug)	1h 15m 21s
aarch64-unknown-linux-gnu/gcc (Release)	37m 5s
aarch64-unknown-linux-gnu/clang (Debug)	1h 8m 2s
aarch64-unknown-linux-gnu/clang (Release)	38m 29s
	7h 7m 38s

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