Convert a character to ASCII lower case.
Unlike the standard C library
tolower() function, this only
recognizes standard ASCII letters and ignores the locale, returning
all non-ASCII characters unchanged, even if they are lower case
letters in a particular character set. Also unlike the standard
library function, this takes and returns a char, not an int, so
don’t call it on
EOF but no need to worry about casting to #guchar
before passing a possibly non-ASCII character in.