Major refactor to use ustring in important places.

Major refactor to use ustring in important places.

There is much more work to do but this converts all the textual I/O, parser
buffers, pathnames, URIs, and a bunch of the XML code.  The older Unicode
support in (runtime unicode) is completely gone now.  Outside of Edwin, it
should be fairly safe to assume that legacy strings are *NOT* UTF-8 encoded.

Some specific work items remaining:

* Eliminate symbol-name, which violates the non-utf8-legacy rule.

* Finish converting the XML code to consistently use ustrings.

* Implement real Unicode casing, ordering, and character sets.

* Change the parser to use the R7RS-defined character classes.

* Isolate Edwin from the runtime system's string implementation, since porting
  it to Unicode is not worth the trouble.  It should be frozen to use only
  ASCII, not ISO 8859-1 as at present.

And last of all:

* Once Edwin is isolated, convert the runtime system to use ustrings everywhere,
  then rename them from "ustring" to "string".