Remove `AbstractMetavarContext` and `TmpMetavarContext`.
Disable `TypeUtil` until we decide this is the right design.
Using very simple approach:
- No distinction betwen temporary and regular metavariables.
- Metavariables have a `depth` Nat field.
- MetavarContext also has a `depth` field.
- We bump the `MetavarContext` depth when we create a nested problem.
Example: Elaborator (depth = 0) -> Simplifier matcher (depth = 1) -> TC (level = 2) -> TC (level = 3) -> ...
- When MetavarContext is at depth N, isDefEq does not assign variables from depth < N.
- Metavariables from depth N+1 must be fully assigned before we return to level N.
- New design even allows us to invoke tactics from TC.
cc @dselsam
e844a5be74