To handle delaborating notations that are functions that can be applied
to arguments, extracts the core function application delaborator as a
separate function that accepts the number of arguments to process and a
delaborator to apply to the "head" of the expression.
Defines `withOverApp`, which has the same interface as the combinator of
the same name from std4, but it uses this core function application
delaborator.
Uses `withOverApp` to improve a number of application delaborators,
notably projections. This means Mathlib can stop using `pp_dot` for
structure fields that have function types.
Incidentally fixes `getParamKinds` to specialize default values to use
supplied arguments, which impacts how default arguments are delaborated.
---------
Co-authored-by: Sebastian Ullrich <sebasti@nullri.ch>
The `infix` declaration was generating a delaborator, but it is
producing the invalid term
```
ConstantFunction.f myFun 3 <|> fun _ => ConstantFunction.f myFun 4
```
It is invalid because the `macro_rules` for `<|>` is based on
`binop_lazy%` which introduces the `fun _ =>` for us.
I tried to use
```
notation:20 a:21 " <|> " b:20 => HOrElse.hOrElse a fun _ : Unit => b
```
but the delaborator generator does not work for this case.
