Bounds check improvements, BoundsCheckError

[inducer]

#162 has made loopy quite strict in its bounds checking. (cf. https://github.com/inducer/meshmode/issues/81) That's probably a good direction, but we need to

• [ ] improve what assume can do in order to counter all those new bounds check failures.
• [ ] make an option to turn bounds checking off, for emergencies.

cc @kaushikcfd