Make it so array containers do not know their array context?

This is aimed particularly at `DOFArray.array_context`, which is AFAIK the only instance of this, but a pervasive one. This change, if implemented, is rather break-the-world-y, so we would have to do this slowly and carefully.

Why is it worth doing? It's forcing `arraycontext.container.traversal.{freeze,thaw}` to be `@singledispatch` over the container type, just for a way to remove the stored `.array_context`. For batch-freezing in the array context, this is inconvenient (see discussion in https://github.com/inducer/arraycontext/pull/158).

What does it break?
* `grudge` gets its array context from the arrays it's passed for many operations.
* Any function that's `actx.compile`d, because that's its sole source of array cotnext.

That seems like a lot of breakage for limited gain...

@alexfikl @kaushikcfd @majosm Thoughts?