Add SslCurve::to_nid() and remove SslCurveId #849

Job	Run time
rustfmt	7s
Test (stable)	2m 12s
clippy	8m 26s
Test (arm-android)	2m 1s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	2m 12s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	5m 16s
Test (arm64-android)	2m 12s
Test (i686-android)	2m 28s
Test features	3m 55s
Test FIPS integration	3m 3s
Test (x86_64-android)	2m 6s
Test (aarch64-ios)	2m 30s
Test (aarch64-ios-sim)	2m 42s
Test (x86_64-ios)	2m 11s
Test (i686-linux)	2m 17s
Test (arm-linux)	2m 20s
Test (aarch64-linux)	2m 18s
Test (arm64-macos)	2m 21s
Test (x86_64-macos)	2m 36s
Test (x86_64-mingw)	7m 9s
Test (i686-msvc)	7m 25s
Test (x86_64-msvc)	5m 25s
	1h 13m 12s

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