double compiler speed

The compare function in the `Headers` module was not short-circuiting upon discovering a non-equal field. Adding the short-circuiting produces roughly a 2x speedup in the LocalCompiler. This result came from profiling the LocalCompiler using the policy that you can find below. The average (n=3) total running before the change was 11.647s, and the total running time after the change was 5.551s.

```
filter ethTyp = 0x806; (filter ethTyp = 0x806; port := host) |
filter not ethTyp = 0x806; (filter ethSrc = ff:ea:bb:ad:ab:ba; port := probe) |
filter not ethSrc = ff:ea:bb:ad:ab:ba;
(filter switch = 4;
 (filter ethDst = 00:00:00:00:00:05; port := 4 |
  (filter ethDst = 00:00:00:00:00:04; port := 4 |
   (filter ethDst = 00:00:00:00:00:03; port := 4 |
    (filter ethDst = 00:00:00:00:00:02; port := 4 |
     filter ethDst = 00:00:00:00:00:01; port := 4))) |
  (filter ethDst = ff:ff:ff:ff:ff:ff or
          not
          (ethDst = 00:00:00:00:00:05 or
           (ethDst = 00:00:00:00:00:04 or
            (ethDst = 00:00:00:00:00:03 or
             (ethDst = 00:00:00:00:00:02 or ethDst = 00:00:00:00:00:01))));
   (filter port = 4; (port := 3 | (port := 2 | port := 1)) |
    (filter port = 3; (port := 4 | (port := 2 | port := 1)) |
     (filter port = 2; (port := 4 | (port := 3 | port := 1)) |
      (filter port = 1; (port := 4 | (port := 3 | port := 2)) | drop)))) |
   filter not (ethSrc = 00:00:00:00:00:05) and
          (not (ethSrc = 00:00:00:00:00:04) and
           (not (ethSrc = 00:00:00:00:00:03) and
            (not (ethSrc = 00:00:00:00:00:02) and
             not (ethSrc = 00:00:00:00:00:01))));
   port := learn)) |
 (filter switch = 3;
  (filter ethDst = 00:00:00:00:00:04; port := 1 |
   (filter ethDst = 00:00:00:00:00:03; port := 4 |
    (filter ethDst = 00:00:00:00:00:02; port := 4 |
     filter ethDst = 00:00:00:00:00:01; port := 4)) |
   (filter ethDst = ff:ff:ff:ff:ff:ff or
           not
           (ethDst = 00:00:00:00:00:04 or
            (ethDst = 00:00:00:00:00:03 or
             (ethDst = 00:00:00:00:00:02 or ethDst = 00:00:00:00:00:01)));
    (filter port = 4; (port := 3 | (port := 2 | port := 1)) |
     (filter port = 3; (port := 4 | (port := 2 | port := 1)) |
      (filter port = 2; (port := 4 | (port := 3 | port := 1)) |
       (filter port = 1; (port := 4 | (port := 3 | port := 2)) | drop)))) |
    filter not (ethSrc = 00:00:00:00:00:04) and
           (not (ethSrc = 00:00:00:00:00:03) and
            (not (ethSrc = 00:00:00:00:00:02) and
             not (ethSrc = 00:00:00:00:00:01)));
    port := learn)) |
  (filter switch = 2;
   (filter ethDst = 00:00:00:00:00:04; port := 4 |
    (filter ethDst = 00:00:00:00:00:03; port := 3 |
     (filter ethDst = 00:00:00:00:00:02; port := 2 |
      filter ethDst = 00:00:00:00:00:01; port := 1)) |
    (filter ethDst = ff:ff:ff:ff:ff:ff or
            not
            (ethDst = 00:00:00:00:00:04 or
             (ethDst = 00:00:00:00:00:03 or
              (ethDst = 00:00:00:00:00:02 or ethDst = 00:00:00:00:00:01)));
     (filter port = 4; (port := 3 | (port := 2 | port := 1)) |
      (filter port = 3; (port := 4 | (port := 2 | port := 1)) |
       (filter port = 2; (port := 4 | (port := 3 | port := 1)) |
        (filter port = 1; (port := 4 | (port := 3 | port := 2)) | drop)))) |
     filter not (ethSrc = 00:00:00:00:00:04) and
            (not (ethSrc = 00:00:00:00:00:03) and
             (not (ethSrc = 00:00:00:00:00:02) and
              not (ethSrc = 00:00:00:00:00:01)));
     port := learn)) |
   (filter switch = 1;
    (filter ethDst = 00:00:00:00:00:04; port := 2 |
     (filter ethDst = 00:00:00:00:00:03; port := 1 |
      (filter ethDst = 00:00:00:00:00:02; port := 1 |
       filter ethDst = 00:00:00:00:00:01; port := 1)) |
     (filter ethDst = ff:ff:ff:ff:ff:ff or
             not
             (ethDst = 00:00:00:00:00:04 or
              (ethDst = 00:00:00:00:00:03 or
               (ethDst = 00:00:00:00:00:02 or ethDst = 00:00:00:00:00:01)));
      (filter port = 3; (port := 2 | port := 1) |
       (filter port = 2; (port := 3 | port := 1) |
        (filter port = 1; (port := 3 | port := 2) | drop))) |
      filter not (ethSrc = 00:00:00:00:00:04) and
             (not (ethSrc = 00:00:00:00:00:03) and
              (not (ethSrc = 00:00:00:00:00:02) and
               not (ethSrc = 00:00:00:00:00:01)));
      port := learn)) |
    drop))))
```