Add a match_ignore_ascii_case! macro that does not allocate.

It should replace `match foo.to_ascii_lower().as_slice() { ...}`

@Manishearth I changed map.get to map.find in the lint to work around an ICE:

    task 'rustc' panicked at 'couldn't find node id 0 in the AST map'

Does this look OK?

components/util/cursor.rs

@@ -19,8 +19,8 @@ macro_rules! define_cursor {
 
         impl Cursor {
             pub fn from_css_keyword(keyword: &str) -> Result<Cursor, ()> {
-                match keyword.to_ascii_lower().as_slice() {
-                    $( concat!($css) => Ok(Cursor::$variant), )+
+                match_ignore_ascii_case! { keyword:
+                    $( concat!($css) => Ok(Cursor::$variant) ),+
                     _ => Err(())
                 }
             }