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"

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);

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)