Fix slightly too eager parsing of aliases

src/bin/tokens.rs

@@ -0,0 +1,17 @@
+extern crate units_rs;
+
+use units_rs::*;
+
+fn main() {
+    use std::io::Read;
+    use std::fs::File;
+
+    let mut f = File::open("units.txt").unwrap();
+    let mut buf = vec![];
+    f.read_to_end(&mut buf).unwrap();
+    let string = String::from_utf8_lossy(&*buf);
+    let mut iter = unit_defs::TokenIterator::new(&*string).peekable();
+    let res = unit_defs::tokens(&mut iter);
+
+    println!("{:#?}", res);
+}

src/unit_defs.rs

@@ -223,6 +223,10 @@ fn parse_unknown(mut iter: &mut Iter) {
 }
 
 fn parse_alias(mut iter: &mut Iter) -> Option<(Expr, String)> {
+    match *iter.peek().unwrap() {
+        Token::Newline | Token::Comment | Token::Eof => return None,
+        _ => ()
+    }
     let expr = parse_expr(iter);
     match iter.next().unwrap() {
         Token::TriplePipe => (),