miniscript: mark nodes with duplicate keys as insane

As stated on the website, duplicate keys make it hard to reason about
malleability as a single signature may unlock multiple paths.

We use a custom KeyCompare function instead of operator< to be explicit
about the requirement.

src/test/fuzz/miniscript.cpp

@@ -47,6 +47,10 @@ struct TestData {
 struct ParserContext {
     typedef CPubKey Key;
 
+    bool KeyCompare(const Key& a, const Key& b) const {
+        return a < b;
+    }
+
     std::optional<std::string> ToString(const Key& key) const
     {
         auto it = TEST_DATA.dummy_key_idx_map.find(key);
@@ -90,6 +94,10 @@ struct ScriptParserContext {
         std::vector<unsigned char> data;
     };
 
+    bool KeyCompare(const Key& a, const Key& b) const {
+        return a.data < b.data;
+    }
+
     const std::vector<unsigned char>& ToPKBytes(const Key& key) const
     {
         assert(!key.is_hash);