If the type error is at an implicit argument, we annotate
application with `pp.explicit := true`
Given the type incorrect definition
```
def f {a b c : α} : a = c :=
Eq.trans (a := a) (b := b = c)
```
We now generate the error
```
error: application type mismatch
@Eq.trans α a (b = c)
argument
b = c
has type
Prop
but is expected to have type
α
```
@Kha Note that we only enable `pp.explicit := true` for the relevant
application. That is, we set `pp.explicit := false` for each children.
Unfortunately, there is a corner case.
```
set_option pp.explicit true
def f {a b c : α} : a = c :=
Eq.trans (a := a) (b := b = c)
```
produces the error
```
error: application type mismatch
@Eq.trans α a (b = c)
argument
@Eq α b c
has type
Prop
but is expected to have type
α
```
The reset `pp.explicit := false` overwrote the user option.
I think the simplest solution is the following
1- The delaborator saves the initial set of Options `Init`
2- When it finds a node annotated with a `pp` options, it only
consider the option if it is not set by `Init`.
What do you think?
