Nullable record pattern

[toots]

This PR adds support for optional methods extraction patterns:

# foo = null()
let { foo? } = ()

# foo = 123
let { foo? } = { foo = 123 }

This ties up with inferred types:

def f(x) =
  let { foo? } = x
  foo
end;;
f : ({foo? : 'a}) -> 'a? = <fun>