Have `rekey` update the keys of the distribution parameters to match the KeyedDistribution

Noted in https://github.com/invenia/KeyedDistributions.jl/pull/15:

> If the parent distribution is constructed with a KeyedArray but the KeyedDistribution is given a new set of keys there is no expectation that these should match, e.g.

```
ka = KeyedArray(ones(3); id=[:a, :b, :c])

kd = KeyedDistribution(MvNormal(ka), [:x, :y, :z])
```

I don't think this explicitly goes against documentation, so not a bug. Maybe against the principles of the package - `AxisKeys` and `AxisSets` don't allow inconsistent keys, but there isn't a direct analogy to this, where some inner type (array) and the outer type should have the same keys but they are able to differ.

In any case, it would be useful to check. We could just add the option for `rekey` to update the "inner" keys to match the "outer" keys.

Start by reading the `rekey` operation and the `KeyedDistribution` construction shown in the issue, including how a `KeyedArray` is used as the parent distribution. Determine how an option should synchronize the inner distribution parameters with the outer keys. Done means `rekey` can make the parameter keys match the `KeyedDistribution` keys without changing the default behavior.

Written by the indexing model from the issue text.