This PR updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
Before this commit, each `isDefEq u v` invocation would fail if there
were pending universe level constraints. This commit, moves the
postponed universe constraints back to the `MetaM` state.
It also adds the combinator
```lean
withoutPostponingUniverseConstraints x
```
which executes `x` and throws an error if there are pending universe
constraints. We use the combinator at `elabApp` and `elabBinders`.
Without this commit, we would fail to elaborate simple terms such as
```lean
Functor.map Prod.fst (x s)
```
because after elaborating `Prod.fst` and trying to ensure its type
match the expected one, we would be stuck at the universe constraint:
```
u =?= max u ?v
```
Another benefit of the new approach is better error messages. Instead
of getting a mysterious type mismatch constraint, we get a list of
universe contraints the system is stuck at.
cc @Kha
