Add Musig2 module #1541

Job	Run time
Test - stable toolchain (minimal)	4m 40s
Prepare	4s
Test - stable toolchain (recent)	3m 55s
Docs - stable toolchain (recent)	20s
Test 32-bit version	30s
Format - nightly toolchain	11s
Test - 1.63.0 toolchain (minimal)	5m 25s
Test - 1.63.0 toolchain (recent)	5m 21s
WASM - stable toolchain	2m 8s
Lint - nightly toolchain (recent)	25s
Bench - nightly toolchain (recent)	34s
Docs - nightly toolchain (recent)	28s
ASAN - nightly toolchain (recent)	1m 39s
Test - nightly toolchain (minimal)	3m 5s
no-std - nightly toolchain (recent)	44s
Test - nightly toolchain (recent)	3m 5s
	32m 34s

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