Record spread and never type

[toots]

This PR does two things:

Add an impossible never type when removing record methods

Example:

def f(x) =
 let y.{foo, gni} = x
 y
end

f : ('c.{foo : 'b, gni : 'a}) -> 'c.{foo? : never, gni? : never} = <fun>

This makes sure that the returned y value never has any foo or gni method, which is the runtime behavior.

Before that, the following was a correct function:

def f(x) =
  let y.{foo} = x
  y.foo + 1
end

f : ('b.{foo : 'a}) -> int = <fun>

Add syntactic sugar for record and record pattern spread

# This:
let y.{foo, gni} = x

# Can be written as:
let {foo, gni, ...y} = x

# This:
y = x.{foo = 123, gni = "aabb"}

# Can be written as:
y = {foo = 123, gni = "aabb", ...x}

These spread patterns are much more common nowadays and will help programmers being more confident in learning and understanding the language.

Ideally, we would like the spread to only apply to records (unit values with methods) and not other values, for instance:

x = 1.{foo = 123}
let { ...y } = x
y : { foo = 123}

However, I don't think that this is currently possible with the existing type system because we need to keep the root type universal in case this becomes wrapped into a function:

def f(x) =
  let {foo, gni, ...y } = x
   y
end

f : ('c.{foo : 'b, gni : 'a}) -> 'c.{foo? : never, gni? : never} = <fun>

There isn't a way currently to both say that y should be a record and that it should retain all the other methods from x. This might be possible with e.g. a constraint but I didn't want to enter that territory yet as I believe non-unit values with methods are still pretty advanced so why not let the advanced programmer know what they are doing?

[smimram]

I think that this is good to go.

Small question: do we want to introduce a syntax to hide fields? Currently, this can be done by matching, but this is a bit heavy...

[toots]

I think that this is good to go.

Small question: do we want to introduce a syntax to hide fields? Currently, this can be done by matching, but this is a bit heavy...

Thank! It'd be nice to have a syntax for it. This could be done is a different PR.