Reorganize the R6RS I/O condition types

Reorganize the R6RS I/O condition types

Move the I/O condition types from `(rnrs conditions)', where they were
not exported, to `(rnrs files)', where they are.

* module/rnrs/conditions.scm: Remove definition of I/O condition types.
* module/rnrs/files.scm: Replace references to I/O condition types
  inside `(rnrs conditions)' with the actual definitions.
* module/rnrs/io/simple.scm: Don't `@@'-reference the I/O condition types, just
  imported them from `(rnrs files)'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>