Improve usage of FixedString #6373

Job	Run time
Check (unstable (no default features))	34s
Check (temp cache)	47s
Check (unstable + typesize)	41s
Check (temp cache + typesize)	46s
Build docs	33s
Check minimal versions	34s
Examples	2m 49s
Test (zlib compression)	1m 0s
Test (no cache)	1m 0s
Test (zlib and zstd compression)	1m 7s
Test (chrono)	40s
Test (macOS)	1m 12s
Test (Windows)	3m 2s
Test (nightly)	47s
Test (zstd compression)	36s
Test (no gateway)	53s
Test (beta)	48s
Test (no default features)	26s
Test (native-tls)	1m 4s
Test (stable)	1m 8s
Check (builder without model)	35s
MSRV	0s
	21m 2s

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