A (un)splat operator for named arguments

What I'd like is for it to behave just like a regular function application, so:

```
local foo(x, y=100, z) = { x: x, y: y, z: z }

local bar = {
x: 1,
y: 2
}

foo(**bar) # Should return { x: 1, y: 100, z: 2 }
```

Going the other way would also be nice:

```
local foo(**kwargs) = kwargs

foo(x=6, y=1) # Should return { x: 6, y: 1 }
```