Improve freeze/thaw usability

[inducer]

In addition to a better explanation of freeze and thaw, this PR makes it possible to fail noisily in situations like https://github.com/inducer/grudge/pull/121#discussion_r648598039. Specifically, the new ArrayContext.clone() method allows making the "setup" array context of the discretization semantically distinct from the array context used in user code, which allows these mistakes to be caught.

This PR comes with additional small changes in meshmode and grudge that make use of these features:

• https://github.com/inducer/meshmode/pull/218
• https://github.com/inducer/grudge/pull/123

@thomasgibson Could you take a look, specifically at the explanation that I've added?

Closes #19. Closes #21.

[inducer]

@anderson2981 @mtcam I recall that both of you also had gripes about the freeze/thaw interface. Please take a look here and see if this helps make this a bit clearer.

[inducer]

I'm starting to like this more and more. It found a few bugs in meshmode, for instance:

• https://github.com/inducer/meshmode/pull/218/checks?check_run_id=2788506274
• https://github.com/inducer/meshmode/pull/218/commits/77342161958a6801e83fa5dcdc30e73fef7bd54a

[inducer]

@alexfikl Thanks for taking a look!