Add module "musig" that implements MuSig2 multi-signatures (BIP 327)

ci/ci.sh

@@ -13,7 +13,7 @@ print_environment() {
     # does not rely on bash.
     for var in WERROR_CFLAGS MAKEFLAGS BUILD \
             ECMULTWINDOW ECMULTGENKB ASM WIDEMUL WITH_VALGRIND EXTRAFLAGS \
-            EXPERIMENTAL ECDH RECOVERY EXTRAKEYS SCHNORRSIG ELLSWIFT \
+            EXPERIMENTAL ECDH RECOVERY EXTRAKEYS MUSIG SCHNORRSIG ELLSWIFT \
             SECP256K1_TEST_ITERS BENCH SECP256K1_BENCH_ITERS CTIMETESTS\
             EXAMPLES \
             HOST WRAPPER_CMD \
@@ -79,6 +79,7 @@ esac
     --enable-module-ellswift="$ELLSWIFT" \
     --enable-module-extrakeys="$EXTRAKEYS" \
     --enable-module-schnorrsig="$SCHNORRSIG" \
+    --enable-module-musig="$MUSIG" \
     --enable-examples="$EXAMPLES" \
     --enable-ctime-tests="$CTIMETESTS" \
     --with-valgrind="$WITH_VALGRIND" \