Add extension.

* f.scm (extension): New procedure.

Add replace-extension.

* f.scm (no-extension): New procedure.
(replace-extension): Likewise.

Remove outdated installation instructions.

Fix modules.

Fix module name in README

More docs.

More stuff in the README.

Complete copy.

Fix some formatting in the README.

Add a bit of documentation.

Add traverse.

Rename (f p) to (f ports).

Simplify filter by using remove.

Add delete.

Add ls.

Add a bit of docs.

Add mkdir.

Re-export some common functions.

Fix mailing list link.

Update README.