Use CStr literals everywhere possible #4413

Job	Run time
clippy-check	2m 10s
smithay-check-features	2m 18s
format	7s
check-minimal	1m 8s
anvil-check-features	2m 25s
Documentation on Github Pages	56s
smithay-tests	3m 19s
check-msrv	1m 5s
smallvil-check	48s
WLCS: Core tests	1m 14s
WLCS: Output tests	1m 15s
WLCS: Pointer input tests	1m 3s
WLCS: Bad Buffer Test	1m 9s
	18m 57s

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