GCC 10 gives -Wabsolute-value warning on hat-trie.c indicating signedness bug

[mjakubicek]

hat-trie.c: In function ‘hattrie_split’:
hat-trie.c:284:18: warning: taking the absolute value of unsigned type ‘unsigned int’ has no effect [-Wabsolute-value]
  284 |         if (d <= abs(left_m - right_m) && left_m + cs[j + 1] < all_m) {
      |                  ^~~

The expression left_m - right_m likely doesn't work as expected -- it may never be negative because of the unsigned operand types, so the abs has no effect and the operational logic (distance between left_m and right_m) doesn't work.