miniscript: restrict multi() usage to P2WSH context

CHECKMULTISIG is disabled for Tapscript. Instead, we'll introduce
a multi_a() fragment with the same semantic as multi().

src/script/miniscript.cpp

@@ -10,6 +10,7 @@
 #include <assert.h>
 
 namespace miniscript {
+
 namespace internal {
 
 Type SanitizeType(Type e) {