Add support for musig() key expressions #1115

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 46s
Compilation for NanoS, X, S+, and Stax (nanosp, $NANOSP_SDK)	1m 42s
Compilation for NanoS, X, S+, and Stax (stax, $STAX_SDK)	1m 53s
Unit test	1m 14s
Tests on mainnet (nanos)	31s
Tests (nanos)	8m 14s
Tests on mainnet (nanox)	35s
Tests (nanox)	8m 58s
Tests (nanosp)	9m 15s
Tests on mainnet (nanosp)	32s
Tests on mainnet (stax)	36s
Tests (stax)	53m 10s
Tests with the JS library	1m 26s
Tests with the Python library and legacy Bitcoin app	46s
Tests for rust client library	1m 38s
	1h 34m 3s

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