Add support for musig() key expressions #1252

Job	Run time
Compilation for NanoS, X, S+, and Stax (nanos, $NANOS_SDK)	1m 47s
Compilation for NanoS, X, S+, and Stax (nanox, $NANOX_SDK)	1m 51s
Compilation for NanoS, X, S+, and Stax (nanosp, $NANOSP_SDK)	1m 51s
Compilation for NanoS, X, S+, and Stax (stax, $STAX_SDK)	1m 57s
Unit test	2m 8s
Tests with the Python library and legacy Bitcoin app	44s
Tests for rust client library	1m 36s
Tests on mainnet (nanos)	29s
Tests with the JS library	1m 50s
Tests on mainnet (nanox)	43s
Tests on mainnet (nanosp)	30s
Tests on mainnet (stax)	51s
	16m 17s

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