linfa-preprocessing::countgrams: RefCell is replaced with OnceLock #1424

Job	Run time
check-nightly-windows-latest	2m 31s
check-wasm-browser	58s
check-stable-ubuntu-latest	46s
check-1.87.0-windows-latest	2m 16s
check-1.87.0-ubuntu-latest	1m 10s
check-stable-windows-latest	2m 40s
check-nightly-ubuntu-latest	1m 4s
	11m 25s

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