Add support for musig() key expressions #1260

Job	Run time
Compilation for NanoS, X, S+, and Stax (nanos, $NANOS_SDK)	1m 45s
Compilation for NanoS, X, S+, and Stax (nanox, $NANOX_SDK)	1m 51s
Compilation for NanoS, X, S+, and Stax (nanosp, $NANOSP_SDK)	1m 47s
Compilation for NanoS, X, S+, and Stax (stax, $STAX_SDK)	1m 55s
Tests with the JS library	1m 15s
Tests on mainnet (nanos)	29s
Tests on mainnet (nanox)	30s
Tests on mainnet (nanosp)	28s
Tests with the Python library and legacy Bitcoin app	45s
Tests on mainnet (stax)	28s
Tests for rust client library	1m 44s
Unit test	1m 51s
	14m 48s

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