Implement & test ECDSA support #161

Job	Run time
Clippy	30s
Clippy	35s
Build+test (stable, ubuntu-latest)	6h 0m 13s
Build+test (stable, ubuntu-latest)	1m 36s
Clang Tidy	7s
Clang Tidy	10s
Build+test (beta, ubuntu-latest)	1m 37s
Build+test (beta, ubuntu-latest)	2h 25m 52s
Clippy nightly (optional)	34s
Clippy nightly (optional)	39s
Build+test (nightly, ubuntu-latest)	1m 36s
Build+test (nightly, ubuntu-latest)	1m 38s
Check for documentation errors	43s
Check for documentation errors	39s
Format	15s
Format	19s
Miri	1m 7s
Miri	1m 21s
Packaging	2m 42s
Packaging	2m 35s
Valgrind	1m 59s
Valgrind	2m 0s
	8h 48m 47s

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