Implement FromDer for Extension #606

Job	Run time
Clippy	19s
Cargo Deny	9s
Format	11s
Check for documentation errors (stable)	12s
Check for documentation errors (beta)	26s
Check for documentation errors (nightly)	19s
Check cross compilation targets	6m 54s
MSRV	1m 48s
Cargo Package	15s
Check semver compatibility	1m 52s
Build+test (--all-features, stable)	39s
Build+test (stable, ubuntu-20.04)	22s
Build+test (--features=alloc, stable, ubuntu-20.04)	23s
Build+test (--no-default-features, stable, ubuntu-20.04)	21s
Build+test (--all-features, --release, stable, ubuntu-20.04)	1m 41s
Build+test (--all-features, nightly, ubuntu-20.04)	31s
Build+test (--all-features, beta, ubuntu-20.04)	31s
Measure coverage	44s
	17m 37s

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