Add SslCurve::to_nid() and remove SslCurveId #848

Job	Run time
rustfmt	6s
clippy	1m 8s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	1m 41s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	6m 54s
Test features	4m 3s
Test (stable)	2m 10s
Test FIPS integration	3m 0s
Test (arm-android)	2m 9s
Test (arm64-android)	2m 13s
Test (i686-android)	1m 57s
Test (x86_64-android)	2m 6s
Test (aarch64-ios)	2m 31s
Test (aarch64-ios-sim)	2m 17s
Test (x86_64-ios)	2m 32s
Test (i686-linux)	2m 31s
Test (arm-linux)	2m 18s
Test (aarch64-linux)	2m 21s
Test (arm64-macos)	2m 4s
Test (x86_64-macos)	2m 7s
Test (x86_64-mingw)	4m 42s
Test (i686-msvc)	6m 36s
Test (x86_64-msvc)	5m 12s
	1h 2m 38s

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