Merge rust-bitcoin/corepc#44: Add `std` feature #79

Job	Run time
Prepare	2s
Test - 1.63.0 toolchain (minimal)	2m 29s
Integration tests - stable toolchain (28_0)	58s
Docs - stable toolchain (recent)	1m 2s
Test - 1.63.0 toolchain (recent)	2m 28s
Integration tests - stable toolchain (27_1)	52s
Test - stable toolchain (minimal)	46s
Integration tests - stable toolchain (27_0)	52s
Test - stable toolchain (recent)	50s
Verify - stable toolchain	16s
Integration tests - stable toolchain (26_2)	57s
Integration tests - stable toolchain (26_1)	53s
Integration tests - stable toolchain (26_0)	55s
Integration tests - stable toolchain (25_2)	1m 5s
Integration tests - stable toolchain (24_2)	1m 2s
Integration tests - stable toolchain (23_2)	1m 2s
Integration tests - stable toolchain (22_1)	52s
Integration tests - stable toolchain (0_21_2)	1m 5s
Integration tests - stable toolchain (0_20_2)	1m 9s
Integration tests - stable toolchain (0_19_1)	53s
Integration tests - stable toolchain (0_18_1)	1m 48s
Integration tests - stable toolchain (0_17_1)	1m 31s
Docs - nightly toolchain (recent)	47s
Format - nightly toolchain	13s
Lint - nightly toolchain (recent)	37s
Test - nightly toolchain (minimal)	46s
Test - nightly toolchain (recent)	44s
	26m 54s

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