Replace StatsBase with our own `Models.fit` and `Models.predict`

[nickrobinson251]

StatsBase is a large and unnecessaary dependency. Users should not need to depend on anything but this package itself. I propose we just define our own fit and predict, which is extended by packages implementing the interface.

[nickrobinson251]

looks like we'd still use StatsBase.uweights 😞

I really wish StatsBase wasn't such a rag bag of random stats functionality

I think it still makes sense for this package to own its own interface, and so have its own fit and predict, but we won't get the advantage of dropping StatsBase unless we also remove uweights from TestUtils

[nickrobinson251]

Alternatively, we could extend the fit and predict functions from much lighter-weight StatsAPI.jl
https://github.com/JuliaStats/StatsAPI.jl