Closes #18.

Using a lookahead size equal to the window size could previously lead to
an infinite loop (discovered and fixed by @unixdj, thanks). There really
isn't any benefit to having a lookahead size that large, as it only
applies when the entire input stream is the same. In all other cases, it
makes the compression ratio worse.

Closes #20.

BUG: When window size equals lookahead, the encoder enters an endless
loop after WINDOW_SIZE bytes are sunk into the buffer, unless at EOF.

Test:
$ echo -n 0123456789abcdefX | ./heatshrink -w 4 -l 4 >/dev/null

Closes #21

Only check for matches that can be improvements.

Only test whether len is > match_maxlen when at least the first byte
matches, and use a pointer to curry in the current offset, saving an
add per compare.

This eliminates more unnecessary compares, speeding things up further.

(Thanks to user jibsen on hacker news for the original suggestion.)

Also, update index type to int16_t.

Eliminate some unnecessary comparisons by making the position in the
index signed, and treating any negative value as not found, not just
((uint16_t)-1). Also, decrease HEATSHRINK_MAX_WINDOW_BITS to 15, so
index values with the highest bit set can be used as working space
for index optimization on the fly (some day).

Use 'if length >= 3' instead of 'length > 2' for the break even point,
sinec it makes the intent clearer, and pull the comparison outside of
the inner loop.

Increase HEATSHRINK_MIN_LOOKAHEAD_BITS to 2, since -l 1 won't compress.
Also, eliminate the corresponding MAX #define, since it has to be <= the
window size anyway.

Use a separate *pt == *pt2 instead of pt[0] == pt2[0] for comparison of the
leading byte, which should allow for better branch prediction.

Eliminate 'needle_index' in the inner loop -- it's only aliasing 'end'.

Also, add comments about the indexing strategy.

This can save a significant amount of time in push_bits, particularly if -w is 8.

1. Instead of storing an offset to the previous matching byte in the
   index, store the next matching byte bigram. Since the compression
   doesn't break even unless at least 3 bytes match, any single-byte
   matches in the index should just be skipped.

   Since this eliminates these false positives in one pass, it also
   removes work that would otherwise be done during every walk of the
   index's offset chain - possibly several times. This greatly
   reduces the time spent in find_longest_match().

   EDIT: This prematurely discards information from the index,
   damaging the overall compression effectiveness. Some of the
   links being skipped are still necessary for other chains, so
   they cannot be eliminated this way. It's still possible dynamic
   programming can be used for this, but the strategy in this
   commit doesn't quite work.

2. Eliminate the test of the first byte, since indexing already
   incorporates this comparison ahead of time.

*comment on rebase*

This eliminates the possibility of infinite loops caused by counters
overflowing and preventing the output buffer full or input exhausted
conditions from being reached.

Since they're only stored as `uint16_t`s, disallow overly large sizes.