Add SslCurve::to_nid() and remove SslCurveId #854

Job	Run time
rustfmt	11s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	1m 47s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	4m 33s
clippy	8m 5s
Test features	6m 47s
Test FIPS integration	3m 11s
Test (stable)	2m 5s
Test (arm-android)	2m 10s
Test (arm64-android)	2m 0s
Test (i686-android)	1m 53s
Test (x86_64-android)	2m 0s
Test (aarch64-ios)	2m 24s
Test (aarch64-ios-sim)	2m 32s
Test (x86_64-ios)	2m 5s
Test (i686-linux)	2m 24s
Test (arm-linux)	2m 20s
Test (aarch64-linux)	2m 24s
Test (arm64-macos)	2m 5s
Test (x86_64-macos)	2m 14s
Test (x86_64-mingw)	6m 47s
Test (i686-msvc)	6m 51s
Test (x86_64-msvc)	5m 56s
	1h 12m 44s

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