improved syntax checking on expr. Laid foundation for expr function.

tests/testwhile.jet

@@ -1,8 +1,8 @@
 #!../jet-2.0
 <jet cgi="false" name1="localname" filterblanklines="true" trimlines="true">
  <set name="ix" value="1" />
- <while expr="$[ix]<=11">
+ <while expr="$[ix] <= 11">
   -->$[ix]<--
-  <set name="ix" expr="$[ix]+1" />
+  <set name="ix" expr="$[ix] + 1" />
  </while>
 </jet>