Try to include tskit-c as a submodule

[molpopgen]

Doing so would simplify upgrading the C components.

[molpopgen]

Clearing from the 0.7.0 milestone. Definitely worth looking into in the future, but low priority for now.

[Unic-X]

Seems like redoing this commit : e724b0b with some changes should resolve this ?

I'll be glad to fix this if its still an open issue.

[Unic-X]

Also this can be extended for tskit/kastore as well right ?

[molpopgen]

Also this can be extended for tskit/kastore as well right ?

This would happen implicitly, as kastore is itself a submodule of tskit-c.