Change iconv procedures to take optional instead of keyword arg

Change iconv procedures to take optional instead of keyword arg

* module/ice-9/iconv.scm (call-with-encoded-output-string):
  (string->bytevector, bytevector->string): Take an optional instead of
  a keyword argument.

* doc/ref/api-data.texi (Representing Strings as Bytes): Adapt docs to
  change, and fix a number of errors.  Thanks to Ludovic Courtès for the
  pointers.

* test-suite/tests/iconv.test ("wide non-ascii string"): Add a test for
  the 'substitute path.