Improve usage of FixedString (#3418) #6374

Job	Run time
Check (unstable + typesize)	55s
Check (temp cache + typesize)	58s
Check minimal versions	59s
Check (unstable (no default features))	42s
Check (builder without model)	44s
Build docs	52s
Test (beta)	43s
Check (temp cache)	48s
Test (Windows)	4m 4s
Test (nightly)	1m 21s
Test (zlib compression)	1m 5s
Test (zstd compression)	1m 12s
Examples	3m 6s
Test (no cache)	1m 15s
Test (stable)	1m 4s
Test (macOS)	1m 30s
Test (native-tls)	1m 8s
Test (no gateway)	1m 15s
Test (no default features)	45s
Test (zlib and zstd compression)	1m 0s
Test (chrono)	50s
MSRV	-9s
	26m 7s

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