API overhaul

[birktj]

I think that the current api of this crate is sort of messy and a overhaul would be nice. I think this crate should be structured for two use cases: a low-level usage of the tiff format with direct control over tags and data and a mid-level baseline tiff implementation with access to properties specified by baseline tiff (and possibly some more). The high-level use case should be covered by the image crate.

Two things that I think would improve the api a lot would be to reorganize all the small helper types so that they fit together well with both the decoder and encoder and clean up decoder api.

I propose something like this for the decoding api:

impl TiffDecoder<R> {
    fn new(...) {...}

    fn next_directory<'a>(&'a mut self) -> TiffResult<Option<Directory<'a>>> {...}
    fn prev_directory<'a>(&'a mut self) -> TiffResult<Option<Directory<'a>>> {...}

    fn next_image<'a>(&'a mut self) -> TiffResult<Option<Image<'a>>> {...}
    fn prev_image<'a>(&'a mut self) -> TiffResult<Option<Image<'a>>> {...}
}

impl DirectoryDecoder {
    fn read_tag(&mut self, tag: Tag) -> Entry {...}
    fn read_data<T: TiffValue>(&mut self, offset: u32, buffer: &mut T) -> TiffResult<()> {...}
}

impl ImageDecoder {
    fn width(&self) -> u32 {...}
    fn height(&self) -> u32 {...}
    fn pixel_format(&self) -> PixelFormat {...}

    fn read_strip(&mut self) -> DecodingResult {...} // ???
    fn reader<'a>(&'a mut self) -> Reader<'a> {...}

    fn resolution_unit(&mut self) -> ResolutionUnit {...}
    fn x_resolution(&self) -> Rational {...}
    fn y_resolution(&self) -> Rational {...}
    // Other baseline tiff properties  ...
}