use utf8-string for encoding and decoding
forward compatibility with new Cabal library
Trac #1266: remove duplicate instance CellLayoutClass IconView
Trac #1265: export constructors of the enumeration type CellRendererMode
don't emit overlapping patterns when generating toEnum
minor drive-by refactoring
bump version number
clean up after bad fix to Trac #1259

A proposed fix to Trac #1259 added a language extension; later, the problem was
fixed in another way that removed the need for that extension, but didn't
remove the extension. This patch removes the extension.