Make skip_whitespace private again

It was never supposed to be part of a public interface of TokenStream,
but it got added during debugging lifetimes issue. This commit finally
reverts it back to what it was supposed to be.

src/main.rs

@@ -282,7 +282,7 @@ impl<'a> TokenStream<'a> {
 		return self.source[self.cursor..].chars();
 	}
 
-	pub fn skip_whitespace(&mut self) {
+	fn skip_whitespace(&mut self) {
 		let mut chars = self.chars().peekable();
 		let mut length = 0;