Compose: add iterator API

Compose: add iterator API

Allow users to iterate the entries in a compose table. This is useful
for other projects which want programmable access to the sequences,
without having to write their own parser.

- New API:
  - `xkb_compose_table_entry_sequence`;
  - `xkb_compose_table_entry_keysym`;
  - `xkb_compose_table_entry_utf8`;
  - `xkb_compose_table_iterator_new`;
  - `xkb_compose_table_iterator_free`;
  - `xkb_compose_table_iterator_next`.
- Add tests in `test/compose.c`.
- Add benchmark for compose traversal.
- `tools/compose.c`:
  - Print entries instead of just validating them.
  - Add `--file` option.
  - TODO: make this tool part of the xkbcli commands.

Co-authored-by: Pierre Le Marre <dev@wismill.eu>
Co-authored-by: Ran Benita <ran@unusedvar.com>
Signed-off-by: Ran Benita <ran@unusedvar.com>