current-filename canonicalizes path, remove add-to-path

current-filename canonicalizes path, remove add-to-path

* module/ice-9/boot-9.scm (current-filename): Canonicalize the path, so
  that the result is independent of the current directory, and so
  that `dirname' can traverse up the file system, as in the
  add-to-load-path example.
  (add-to-path): Remove.  The eval-when semantics make this macro too
  tricky to explain -- people will start using it on other path-like
  things, in lexical contours, whereas it only really makes sense with
  load-like paths at the toplevel.

* doc/ref/api-evaluation.texi (Loading): Fix link to load-with-path
  docs, and remove add-to-path docs.