add current-filename, add-to-path, add-to-load-path

add current-filename, add-to-path, add-to-load-path

* module/ice-9/boot-9.scm (current-filename, add-to-path)
  (add-to-load-path): New syntaxen.

* doc/ref/api-evaluation.texi (Loading): Move load-path related
  procedures to a new section:
  (Load Paths): Hither.  Document add-to-path and add-to-load-path.

* doc/ref/api-debug.texi (Source Properties): Document
  current-source-location and current-filename.

* doc/ref/api-modules.texi:
* doc/ref/guile-invoke.texi:
* doc/ref/scheme-using.texi: Update @ref for Load Paths change.