Does optimisation result really need to split varnames up?

[penelopeysm]

Right now the results from an Optimisation seem to split vector-valued parameters up into constituent elements (because they use Turing.Inference.getparams), and to avoid causing breaking changes I've preserved this behaviour in #2632 (thus even though getparams now returns a dict we still break it up using varname_and_value_leaves).

But really we only do this because of MCMCChains. It seems quite unnecessary to do it for optimisation results.