Internals: Use magic string for warnMore so lock-free.

src/V3String.h

@@ -126,6 +126,8 @@ public:
     static string::size_type leadingWhitespaceCount(const string& str);
     // Return double by parsing string
     static double parseDouble(const string& str, bool* successp);
+    // Replace substring. Often replaceWord is more appropriate.
+    static string replaceSubstr(const string& str, const string& from, const string& to);
     // Replace all occurrences of the word 'from' in 'str' with 'to'. A word is considered
     // to be a consecutive sequence of the characters [a-zA-Z0-9_]. Sub-words are not replaced.
     // e.g.: replaceWords("one apple bad_apple", "apple", "banana") -> "one banana bad_apple"