Implement FromDer trait and DerIterator type #575

Job	Run time
Clippy	23s
Measure coverage	40s
Check cross compilation targets	48s
Cargo Deny	1m 20s
MSRV	1m 10s
Cargo Package	15s
Check for documentation errors (stable)	9s
Check for documentation errors (beta)	24s
Check for documentation errors (nightly)	23s
Format	5s
Check semver compatibility	1m 13s
Build+test (--all-features, stable)	31s
Build+test (stable, ubuntu-20.04)	23s
Build+test (--features=alloc, stable, ubuntu-20.04)	25s
Build+test (--no-default-features, stable, ubuntu-20.04)	27s
Build+test (--all-features, --release, stable, ubuntu-20.04)	2m 8s
Build+test (--all-features, nightly, ubuntu-20.04)	40s
Build+test (--all-features, beta, ubuntu-20.04)	27s
	11m 51s

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