Pointer fixes/improvements.

Allow <, <=, >, >= comparisons of pointers
Allow explicit type-casting of pointers to and from integers
Fix bug in handling expressions of the form "int + ptr" ("ptr + int"
  was fine).
Fix a bug in TypeCastExpr where varying -> uniform typecasts
  would be allowed (leading to a crash later)

tests_errors/func-export-task.ispc

@@ -0,0 +1,4 @@
+// Function can't have both "task" and "export" qualifiers 
+
+export task void foo() {
+}