Refactor DER parsing #615

Job	Run time
Cargo Deny	10s
Cargo Deny	10s
Clippy	2m 50s
Clippy	31s
MSRV	1m 41s
MSRV	1m 41s
Cargo Package	20s
Cargo Package	20s
Check for documentation errors (stable)	39s
Check for documentation errors (stable)	39s
Check for documentation errors (beta)	37s
Check for documentation errors (beta)	37s
Check for documentation errors (nightly)	25s
Check for documentation errors (nightly)	25s
Format	6s
Format	6s
Measure coverage	40s
Measure coverage	40s
Check cross compilation targets	43s
Check cross compilation targets	43s
Check semver compatibility	1m 13s
Check semver compatibility	1m 13s
Build+test (--all-features, stable)	19s
Build+test (--all-features, stable)	19s
Build+test (stable, ubuntu-20.04)	53s
Build+test (stable, ubuntu-20.04)	53s
Build+test (--features=alloc, stable, ubuntu-20.04)	24s
Build+test (--features=alloc, stable, ubuntu-20.04)	24s
Build+test (--no-default-features, stable, ubuntu-20.04)	29s
Build+test (--no-default-features, stable, ubuntu-20.04)	29s
Build+test (--all-features, --release, stable, ubuntu-20.04)	2m 30s
Build+test (--all-features, --release, stable, ubuntu-20.04)	2m 30s
Build+test (--all-features, nightly, ubuntu-20.04)	32s
Build+test (--all-features, nightly, ubuntu-20.04)	32s
Build+test (--all-features, beta, ubuntu-20.04)	26s
Build+test (--all-features, beta, ubuntu-20.04)	26s
	27m 35s

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