Improve usage of FixedString #6369

Job	Run time
Check (builder without model)	33s
Check (temp cache)	54s
Check (unstable + typesize)	50s
MSRV	0s
Check (temp cache + typesize)	59s
Check (unstable (no default features))	31s
Check minimal versions	41s
Examples	2m 55s
Build docs	51s
Test (macOS)	1m 8s
Test (Windows)	2m 50s
Test (beta)	45s
Test (nightly)	1m 13s
Test (no gateway)	1m 7s
Test (native-tls)	58s
Test (no cache)	1m 12s
Test (stable)	1m 10s
Test (no default features)	40s
Test (zstd compression)	1m 7s
Test (chrono)	38s
Test (zlib and zstd compression)	1m 5s
Test (zlib compression)	57s
	23m 4s

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