Add rerun_except crate, and small example code

* rerun_except will ensure a build doesn't rerun if a *.not file is
  modified, which it would otherwise do
* Add examples/expr.not with some basic assignment statements and
  function calls

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/expr.not

@@ -0,0 +1,5 @@
+kilo = pow(2, 10)
+mega = pow(2, 20)
+giga = pow(2, 30)
+tera = pow(2, 40)
+peta = pow(2, 50)