Array context

In addition to removing the reshape-based DOF vector indexing, this works towards making the arrays in meshmode abstract enough to allow lazy eval (i.e. inducer/pytato) to be used.

WIP because:

• ⚠ Re-dangle requirements.txt (will do on master, after the pytential bit of this lands)
• Should go in at the same time as pytential!198 (merged)
• Should go in at the same time as grudge!71 (merged)