Add HasCanonicalPushes(), and use it in IsStandardTx

src/script.h

@@ -541,9 +541,12 @@ public:
 
     bool IsPayToScriptHash() const;
 
-    // Called by IsStandardTx
+    // Called by IsStandardTx and P2SH VerifyScript (which makes it consensus-critical).
     bool IsPushOnly() const;
 
+    // Called by IsStandardTx.
+    bool HasCanonicalPushes() const;
+
     // Returns whether the script is guaranteed to fail at execution,
     // regardless of the initial stack. This allows outputs to be pruned
     // instantly when entering the UTXO set.