GIF: rename PrefixTable -> ForwardPrefixTable

ReversePrefixTable will be along later.
GIF: decoding exceptions
BitReader: better EOF handling
BitReader: debug tests

Not sure how that got through.
ReversePrefixTable tests
BitReader: fix Clear
GIF: basic loading

Not very well tested yet.
GIF: add a test for the tricky LZW case
GIF: miscellaneous bugfixes