Implement reverse(str) and concat(a, b, ...) in the expr engine

Both matched their function name but had empty branch bodies, silently
producing nothing regardless of arguments. reverse(str) follows the same
single-argument shape as trim/toupper/tolower; concat(a, b, ...) follows
the same variadic comma-separated shape as max/min.

reverse() converts to std::string and reverses in place via the already-
imported <algorithm> (std::reverse), then assigns back through MString's
std::string constructor. concat() starts from the first argument and
appends each subsequent one via MString's own operator<<(ZString).

Verified via real jet-2.0 runs: reverse('hello') -> "olleh",
concat('a','b','c') -> "abc", concat('x','y') -> "xy". Full 9-script
regression suite (custom tag scoping, dump tag tree, try/catch, if/else,
keyword sigil) still passes unchanged.

Operand.cpp

@@ -124,9 +124,26 @@ namespace jet {
 	 } else
 	   throw coreutils::Exception("Expecting ) at end of TOLOWER expression.");	 	 	 	 	 
       } else if(in.ifNextIgnoreCase("REVERSE")) {
-	 
+	 if(!in.ifNext("("))
+	   throw coreutils::Exception("Expecting ( for REVERSE parameters.");
+	 Operand parm1(in, tag);
+	 if(in.ifNext(")")) {
+	    std::string reversed = parm1.string.str();
+	    std::reverse(reversed.begin(), reversed.end());
+	    string = reversed;
+	 } else
+	   throw coreutils::Exception("Expecting ) at end of REVERSE expression.");
       } else if(in.ifNextIgnoreCase("CONCAT")) {
-	 
+	 if(!in.ifNext("("))
+	   throw coreutils::Exception("Expecting ( for CONCAT parameters.");
+	 Operand parm1(in, tag);
+	 string = parm1.string;
+	 while(in.ifNext(",")) {
+	    Operand parm(in, tag);
+	    string << parm.string;
+	 }
+	 if(!in.ifNext(")"))
+	   throw coreutils::Exception("Expecting ) at end of CONCAT expression.");
       } else if(in.ifNextIgnoreCase("INTEGER")) {
 	 if(!in.ifNext("("))
 	   throw coreutils::Exception("Expecting ( for INTEGER parameters.");