New loop nest constraints (newest nest-constraint PR, directly against main)

[jdsteve2]

Specify and check must-nest and must-not-nest constraints on loop nestings.

• [x] Constraint specification and checking
• [x] Enforcement during linearization
• Child PR covers processing of constraints during transformations: Handle new loop nest constraints during transformation

Older, out-of-date PRs with this content:

• Original PR branching off dependency semantics stuff that includes enforcement of constraints during many (not all) transformations
• Second PR made against @kaushikcfd's new scheduling stuff (PR against main was initially made by moving those commits here)

CC @kaushikcfd