Converts a string into a form that is independent of case.
The result will not correspond to any particular case, but can be compared for equality or ordered with the results of calling casefold on other strings.
Note that calling casefold followed by collate is only an approximation to the correct linguistic case insensitive ordering, though it is a fairly good one. Getting this exactly right would require a more sophisticated collation function that takes case sensitivity into account. GLib does not currently provide such a function.
a UTF-8 encoded string
a newly allocated string, that is a case independent form of