Tighten up string type representations to prevent illegal values #875

Job	Run time
Format	3s
build (macOS-latest, stable)	1m 1s
build-windows	14m 17s
build (ubuntu-latest, stable)	41s
Build rustls-cert-gen as a standalone package	29s
build (ubuntu-latest, beta)	42s
build (ubuntu-latest, nightly)	42s
Validate external types appearing in public API	1m 18s
Clippy (--all-features)	4m 56s
build (ubuntu-latest, stable 7 months ago)	52s
Clippy (--no-default-features --features ring)	4m 48s
Clippy (--no-default-features --features aws_lc_rs)	5m 15s
Measure coverage	5m 35s
Clippy (--no-default-features --features aws_lc_rs,pem)	4m 40s
Documentation (stable)	36s
Documentation (beta)	44s
Check semver compatibility	32s
Documentation (nightly)	43s
Documentation (stable 7 months ago)	52s
	48m 46s

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