Add support for musig() key expressions #1125

Job	Run time
Compilation for NanoS, X, S+, and Stax (nanos, $NANOS_SDK)	1m 42s
Compilation for NanoS, X, S+, and Stax (nanox, $NANOX_SDK)	1m 47s
Compilation for NanoS, X, S+, and Stax (nanosp, $NANOSP_SDK)	1m 51s
Compilation for NanoS, X, S+, and Stax (stax, $STAX_SDK)	2m 3s
Unit test	1m 51s
Tests (nanos)	8m 16s
Tests with the Python library and legacy Bitcoin app	46s
Tests (nanox)	9m 1s
Tests on mainnet (nanos)	37s
Tests (nanosp)	9m 8s
Tests with the JS library	1m 28s
Tests (stax)	21m 57s
Tests on mainnet (nanox)	30s
Tests for rust client library	1m 39s
Tests on mainnet (nanosp)	35s
Tests on mainnet (stax)	34s
	1h 3m 45s

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