bug#38235: string-foldcase bug for trailing sigma

[Andy Wingo]

Given the following example, using (rnrs unicode):

  (string-foldcase "ΜΈΛΟΣ")

The expected result is "μέλοσ"; see R6RS libraries section 1.2.  However
instead Guile's result is "μέλος".  Note that although Σ usually
downcases to σ, at the end of a string it's ς.  This test shows a
limitation of defining string-foldcase as simply (string-downcase
(string-upcase str)).