Implement the `explain` function

[green-coder]

The explain function should indicate the reason why a data would not match a model.

As Minimallist is less and less experimental, used by Vrac on large models, this function really start to become important.

[green-coder]

There is a problem .. on large models with a lot of :alt nodes, the error message will be huge and won't be helpful.

If we want to provide a useful error on the :alt node, we need an oracle which will say which branch of the model is the closest to match a value.

We might consider letting the user specify such oracle as an attribute of the :alt node.