Codecs for major file types

[lmmx]

Some sort of 'codec' (i.e. classes to read RangeStreams for certain file types) for:

• [x] zip (regular, conda)
  • of interest for impscan (conda listings) as discussed in issue 15
  • unclear why central directory is not the size indicated by its struct
• [x] zstd
• [ ] tarball (gzip, bz2)
• [x] png
  • critical chunks, first of which is IHDR (width, height, depth, colour type, compression method, ...)
  • of interest for wikitransp dataset preparation (WIT is many TB on disk, 25 GB of TSV, could efficiently prepare sub-table from sniffing PNG with transparency)

Could break into a range-streams-codecs package but no real reason to prefer multiple small packages over a single one if the goals are cohesive. Instead, make a codecs subpackage (consider refactoring prior work into a subpackage perhaps, later)

[lmmx]

Not implementing compressed tarball codec as solid compression is by nature incompatible with range streaming (the entire tarball is compressed, so it's not possible to extract a single file like it is for zip archives, where files are compressed individually).

In that case, all codecs are covered, just need to enhance coverage of other compression formats as needed