ENH: minor improvements, cleanup of token class

- relax casting rules
  * down-cast of labelToken to boolToken
  * up-cast of wordToken to stringToken.
    Can use isStringType() test for word or string types

- simplify constructors, move construct etc.

- expose reset() method as public, which resets to UNDEFINED and
  clears allocated storage etc.

DEFEATURE: remove assign from word or string pointer.

- This was deprecated 2017-11 and now removed.
  For this type of content transfer, move assignment should be used
  instead of stealing pointers.

applications/test/dictionaryTokens/dictionaryTokens.C

@@ -34,7 +34,7 @@ Foam::token Foam::dictionaryTokens::keywordToken(const entry& e)
 
     if (k.empty())
     {
-        return token::undefinedToken;
+        return token();
     }
     if (k.isPattern())
     {