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>

Cargo.toml

@@ -10,6 +10,7 @@ edition = "2018"
 cfgrammar = "0.9"
 lrpar = "0.9"
 lrlex = "0.9"
+rerun_except = "0.1.2"
 
 [dependencies]
 cfgrammar = "0.9"