compose: use a ternary tree instead of a regular trie

Previously we used a simple trie with a linked list for each chain.
Unfortunately most compose files have very long chains which means the
constructions performs an almost quadratic number of comparisons.

Switch to using a ternary search tree instead. This is very similar to a
trie, only the linked list is essentially replaced with a binary tree.

On the en_US/Compose file, the perf diff is the following (the modified
function is `parse`):

  Event 'cycles:u'

  Baseline  Delta Abs  Shared Object     Symbol
  ........  .........  ................  .................................

    39.91%    -17.62%  bench-compose     [.] parse.constprop.0
    20.54%     +6.47%  bench-compose     [.] lex
    17.28%     +5.55%  libc-2.33.so      [.] __strcmp_avx2
    12.78%     +4.01%  bench-compose     [.] xkb_keysym_from_name
     2.30%     +0.83%  libc-2.33.so      [.] __GI_____strtoull_l_internal
     3.36%     +0.78%  bench-compose     [.] strcmp@plt

Thanks to some careful packing, the memory usage is pretty much the
same.

Signed-off-by: Ran Benita <ran@unusedvar.com>

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