Implementation and test cases for R6RS (rnrs files) library.

Implementation and test cases for R6RS (rnrs files) library.

* module/Makefile.am: Add rnrs/6/files.scm to RNRS_SOURCES.
* module/rnrs/6/conditions.scm (define-condition-type): Use specified
  accessor name to create accessor binding.  Add internally-visible
  &i/o-* condition types.
* module/rnrs/6/files.scm: New file.
* module/rnrs/io/6/simple.scm: Export &i/o-* condition types clandestinely
  imported from (rnrs conditions).
* test-suite/Makefile.am: Add tests/r6rs-files.test to SCM_TESTS.
* test-suite/test/r6rs-files.test: New file.