Deprecate CArray?

Behaviour-wise, `CArray` is functionally identical to `Array`. The only difference is that there's an attribute `coerce` with value `True` for `CArray` and `False` for `Array`; that value has no effect on the behaviour of either class.

I suspect that `CArray` is little-used within real code. We could consider deprecating it and recommending that users use `Array` instead.

For some history about how things ended up this way, see #547.