ctutils: split `CtAssign` out of `CtSelect` [BREAKING] #1585

Job	Run time
macos (stable)	22s
cross (aarch64-unknown-linux-gnu, stable)	38s
build-only	18s
windows (x86_64-pc-windows-gnu, 1.85.0)	43s
macos (1.85.0)	22s
windows (x86_64-pc-windows-gnu, stable)	39s
cross (aarch64-unknown-linux-gnu, 1.85.0)	38s
minimal-versions / minimal-versions	21s
linux (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	34s
linux (x86_64-unknown-linux-gnu, 1.85.0)	16s
linux (x86_64-unknown-linux-gnu, stable)	14s
linux (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	33s
	5m 38s

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