Add TryFrom from &[u8] and Vec<u8> to PrivateKeyDer #337

Job	Run time
Smoke-test fuzzing targets	1m 3s
Build + test (stable, ubuntu-20.04)	16s
Clippy	14s
Format	16s
Build + test (beta, ubuntu-20.04)	17s
Build + test (nightly, ubuntu-20.04)	13s
msrv (ubuntu-20.04)	45s
Check semver compatibility	2m 28s
Build + test (macos-latest, stable)	23s
Build wasm32	22s
msrv (macos-latest)	1m 5s
Build + test (windows-latest, stable)	47s
msrv (windows-latest)	1m 13s
	9m 22s

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