Eliminate inefficient use of ArrayList.remove in Scanner

When performing lookahead, the Scanner caches Tokens in an ArrayList. Then, to advance, it removes each token, in turn, from the _front_ of the ArrayList, triggering an array copy to shift every list element to the left.

if (!savedTokens.isEmpty()) token = savedTokens.remove(0);

This probably has no noticeable impact (lookaheads tend to be small, and scanning is IO-bound), but is still totally unnecessary. An ArrayDeque, which has a constant-time 'removeFirst' but otherwise shares most of the characteristics of ArrayList, would be more appropriate.