- closes#1991
This pr implements named arguments as described in #1991. It does not
yet implement optional arguments, which should be added in a later pr as
they are not required for record syntax.
# Syntax Overview
Named arguments are a convenient mehcanism to provide arguments, where
we give the arguments by name instead of by position. Anything with a
type signature can have named arguments, i.e. functions, types,
constructors and axioms.
For instance, if we have (note that named arguments can also appear on
the rhs of the `:`):
```
fun : {A B : Type} (f : A -> B) : (x : A) -> B := ... ;
```
With the traditional positional application, we would write
```
fun suc zero
```
With named arguments we can write the following:
1. `fun (f := suc) (x := zero)`.
2. We can change the order: `fun (x := zero) (f := suc)`.
3. We can group the arguments: `fun (x := zero; f := suc)`.
4. We can partially apply functions with named arguments: `fun (f :=
suc) zero`.
5. We can provide implicit arguments analogously (with braces): `fun {A
:= Nat; B := Nat} (f := suc; x := zero)`.
6. We can skip implicit arguments: `fun {B := Nat} (f := suc; x :=
zero)`.
What we cannot do:
1. Skip explicit arguments. E.g. `fun (x := zero)`.
2. Mix explicit and implicit arguments in the same group. E.g. `fun (A
:= Nat; f := suc)`
3. Provide explicit and implicit arguments in different order. E.g. `fun
(f := suc; x := zero) {A := Nat}`.
