First pass at file locator.

Major things that need to be done:
  1) Extend trove/file.arr to have utilities for operating on paths
  2) Should the argument to the locator involve compile-contexts and a base
     file instead of the whole path? Seems so given that we have
     update-compile-context for handling relative references.
  3) Actually store the compiled version in the filesystem  and/or retrieve
     it if it exists (currently we do the safe thing of not caching).