This PR causes Lean to search through `@[suggest_for]` annotations on
certain errors that look like unknown identifiers that got incorrectly
autobound. This will correctly identify that a declaration of type
`Maybe String` should be `Option String` instead.
## Example
```
example : Except String Unit := return .ok ()
```
```
Function expected at
Result
but this term has type
?m.1
Note: Expected a function because this term is being applied to the argument
String
Hint: The identifier `Result` is unknown, and Lean's `autoImplicit` option causes an unknown identifier to be treated as an implicitly bound variable with an unknown type. However, the unknown type cannot be a function, and a function is what Lean expects here. This is often the result of a typo or a missing `import` or `open` statement.
Perhaps you meant `Except` in place of `Result`?
```
The last line is added by this PR.
This PR provides an additional hint when the type of an autobound
implicit is required to have function type or equality type — this
fails, and the existing error message does not address the fact that the
source of the error is an unknown identifier that was automatically
bound.
## Example
```
import Lean
example : MetaM String := pure ""
```
Current error message:
```
Function expected at
MetaM
but this term has type
?m
Note: Expected a function because this term is being applied to the argument
String
```
Additional error message provided by this PR:
```
Hint: The identifier `MetaM` is unknown, and Lean's `autoImplicit` option
causes an unknown identifier to be treated as an implicitly bound variable
with an unknown type. However, the unknown type cannot be a function, and a
function is what Lean expects here. This is often the result of a typo or a
missing `import` or `open` statement.
```
