file names and comments are now MaybeUTF8 (not String).

ZIP does not really support a UTF-8 file name and comment unless
there is an explicit, rarely present flag set, hence the change.

for the convenience, file search (`get_file_info`) can be done with
normal string so the simple ASCII path would continue to work.
also, MaybeUTF8 itself contains various methods to convert to
Vec<u8> or String (the latter assuming UTF-8 or user-supplied encoding).