Add fast operator evaluation for tensor-product discretizations

[a-alveyblanc]

Supersedes #313.

Adds:

• Fast operator evaluation for tensor-product discretizations
• Metadata (tags) relevant to tensor-product discretizations used during (eager and lazy) compilation

Non-essential transformations will be added in a later PR. This is to keep this (already large) PR manageable.

TODOs:

• [x] Gradient
  • [x] Strong form
  • [x] Weak form
• [x] Divergence
  • [x] Strong form
  • [x] Weak form
• [x] Mass
• [x] Inverse mass
• [ ] Face mass
• [x] Support overintegration + fast operator evaluation

cc @MTCam

[inducer]

Should this be marked draft given that there are pending TODOs?