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>

build.rs

@@ -1,8 +1,13 @@
 use cfgrammar::yacc::YaccKind;
 use lrlex::LexerBuilder;
 use lrpar::CTParserBuilder;
+use rerun_except::rerun_except;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Skip rerunning if it's just *.not files that have changed
+    rerun_except(&["*.not"]).unwrap();
+
+    // Build parser + lexer
     let mut parser_builder = CTParserBuilder::new()
         .yacckind(YaccKind::Grmtools)
         .error_on_conflicts(false);