R4RS 6.7  (string=? string1 string2)     ==>  boolean
          (string-ci=? string1 string2)  ==>  boolean

Returns #T if the two strings are the same length and contain the
same characters in the same positions, otherwise returns #F.
STRING-CI=? treats upper and lower case letters as though they were
the same character, but STRING=? treats upper and lower case as
distinct characters.
