Add Musig2 module #1540

Job	Run time
Docs - stable toolchain (recent)	14s
Test - stable toolchain (minimal)	38s
Prepare	2s
Test - stable toolchain (recent)	48s
Test 32-bit version	27s
Format - nightly toolchain	15s
Test - 1.63.0 toolchain (minimal)	1m 39s
Test - 1.63.0 toolchain (recent)	1m 50s
WASM - stable toolchain	1m 49s
Docs - nightly toolchain (recent)	18s
Test - nightly toolchain (minimal)	41s
ASAN - nightly toolchain (recent)	1m 23s
Test - nightly toolchain (recent)	41s
Bench - nightly toolchain (recent)	21s
Lint - nightly toolchain (recent)	22s
no-std - nightly toolchain (recent)	40s
	12m 8s

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