Improve performance of static :import candidate search

Changes:

    - search/available-classes contains a seq of class symbols, not
      strings

    - A new var, search/available-classes-by-last-segment, contains the
      list above grouped by last segment

    - :import static candidate search is now O(1), from O(n)

Every :import candidate search triggered a string/split + compare for
every known static class. Within slamhound for instance, this is 32652
iterations per import.

Reducing each of these to a single map lookup is a significant
performance boost for namespaces that contain many imports.