Deprecate generic seekend method

[nhz2]

As noted by @jakobnissen in #149 (comment) and @Marlin-Na in #109 there is no way to implement a generic seekend method that doesn't process the whole file.

This is because after calling seekend on a decompressing stream, position should return the uncompressed data size.

I propose deprecating the generic seekend method and removing it in the next breaking release.

Another option would be to implement a generic seek and seekend using the generic seekstart and skip, but as noted in #149 (comment) this could be surprising as seek is expected to be reasonably fast.