Preparations for the Wordgraph version + minor fixes.

    - Rearrange header file order.
    - Validate that the words are not already split morphemes, to prevent an
      infinite loop. This is still not needed now, but is needed in the Wordgraph
      version.
    - Use the suffix argument of add_alternative() for automatic INFIX_MARK
      prepending.
    - int -> size_t for some variables.
    - Fix whitespace.
    - Fix typos.
    - Adapt the INFIX_MARK macro to its recent definition change.