move reference_sequence to metadata?

[petrelharp]

Some of the ugliness we currently have has to do with preserving the reference sequence, which recall is stored as a separate column in the kastore. However, if it was just top-level metadata, then we'd get it passed around for free, and we could get rid of a number of ugly things, for instance, the need for a wrapper on simplify. However, this would require a change in how SLiM writes it out as well.

However, maybe we're just going to put support for reference sequences into tskit; if so, we should just wait for that. If not, we should definitely move it to metadata.

[petrelharp]

Relies on MesserLab/SLiM#180

[benjeffery]

Discussion of ref seq in tskit is here: tskit-dev/tskit#146 It seems overdue to add tskit support for the reference sequence.