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

tests/testexpr.jet

@@ -0,0 +1,6 @@
+#!../jet-2.0
+<jet name1="localname" filterblanklines="true" trimlines="true">
+<set name="test1" value="5 + 5" />
+<set name="test2" expr="expr($[test1])" />
+$[test2]
+</jet>