Fixed a few bugs, unit tests.

* Added vector_from_array() constructor.
* Added stdvec_copy_append_all().
* Fixed: set.c used compar even if NULL, using default compar l-r then
  now.
* Fixed: vector_sub() had wrong size argument for memcmp().
* Fixed: vector_copy_range() could have set size=0.
* Fixed: ground_box() dropped each literal's old z.
* Fixed: hplus() used n_query_vars instead of max_vars.
* Todo: Rename univ_clause_t to univ_box_clause_t and add a new
  univ_clause_t for the static part of BATs, which must be grounded,
  too.