Merge rust-bitcoin/rust-secp256k1#750: release secp256k1 0.30.0 #1468

Job	Run time
Test 32-bit version	53s
Docs - stable toolchain (recent)	23s
Format - nightly toolchain	11s
Test - 1.63.0 toolchain (minimal)	4m 49s
Test - 1.63.0 toolchain (recent)	4m 50s
Prepare	2s
Test - stable toolchain (minimal)	3m 29s
WASM - stable toolchain	1m 47s
Test - stable toolchain (recent)	3m 20s
ASAN - nightly toolchain (recent)	1m 29s
Docs - nightly toolchain (recent)	23s
Lint - nightly toolchain (recent)	46s
Bench - nightly toolchain (recent)	26s
Test - nightly toolchain (minimal)	2m 48s
no-std - nightly toolchain (recent)	1m 2s
Test - nightly toolchain (recent)	2m 42s
	29m 20s

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