Refactor column transforms

[benjeffery]

Columns that need special treatment (strings and byte fields) are currently special-cased by name. This is brittle as we have to update tszip every time tskit changes its column and attribute specification. It would be better to store the type information in zarr attrs to use on decode. This would be a new file format, but could be made backwards compatible.

[jeromekelleher]

Marking this for the 0.3.0 version, since there will be a protocol change (but backwards compatible, as you say).