Implement FromDer for Extension #604

Job	Run time
Clippy	29s
Cargo Deny	9s
Check for documentation errors (stable)	17s
Check for documentation errors (beta)	26s
Check for documentation errors (nightly)	20s
Format	18s
Cargo Package	31s
Check cross compilation targets	46s
MSRV	1m 18s
Check semver compatibility	1m 29s
Build+test (--all-features, stable)	19s
Build+test (stable, ubuntu-20.04)	37s
Build+test (--features=alloc, stable, ubuntu-20.04)	27s
Build+test (--no-default-features, stable, ubuntu-20.04)	27s
Build+test (--all-features, --release, stable, ubuntu-20.04)	1m 46s
Build+test (--all-features, nightly, ubuntu-20.04)	34s
Build+test (--all-features, beta, ubuntu-20.04)	35s
Measure coverage	49s
	11m 37s

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