Rewrite I/O string implementation to eliminate coding on strings --

Rewrite I/O string implementation to eliminate coding on strings --
input from a string is just the characters in the string.  The old
string I/O is renamed to refer to byte vectors, and a collection of
convenience procedurs that use byte-vector I/O to do UTF-xx coding.
Additionally, rewrite unicode support to use these I/O ports to do
format conversions.