Fix issue with STRING subtypes.

Which allows (parse-timestring (format-timestring nil (now))) to work
again (tested on SBCL 1.1.6.12, CLISP 2.48) where it previously raised
an error on SBCL because FORMAT-TIMESTRING returns a SIMPLE-BASE-STRING:

debugger invoked on a TYPE-ERROR in thread
  The value "2013-04-16T23:44:28.910375+02:00"
  is not of type
    (SIMPLE-ARRAY CHARACTER (*)).

Since the UPGRADED-ARRAY-ELEMENT-TYPE for BASE-CHAR and CHARACTER may
differ, (SUBTYPEP 'SIMPLE-STRING '(SIMPLE-ARRAY CHARACTER (*))) can be
true or false between implementations, so %SPLIT-TIMESTRING has to
use SIMPLE-STRING (and CHARACTER).