Allow shorthand object literals

[jcubic]

It would be nice to be able to use syntax like this:

(let ((x 1) (y 2))
  &(:x :y))

It may require a bit of work because right now object-expander uses pair-map that iterate over pairs and check if they are key? symbols. There is a need for a more general iteration that is key? aware and skip if there are no values.

This will be important to implement a complex let-bind macro ##184.

[jcubic]

It seems this is impossible to do. Because & is a parser extension and expands on parse time into an object. And x y are runtime values.

The only thing that can be done is to allow undefined values, it probably is only useful with let-bind because object literals are read-only.

[jcubic]

Now shorthand object literals are never read-only:

(let ((obj &(:x :y)))
  (set! obj.x 10)
  (set! obj.y 20)
  obj)

[jcubic]

There is one problematic unit test:

(test "core: it should throw when change object literals long property after short property"
      (lambda (t)
        (let ((obj &(:x :y 20)))
          (set! obj.x 10)
          (t.is (to.throw (set! obj.y 30)) true)
          (t.is obj &(:x 10 :y 20)))))

if the first property is short the object is not frozen so you can still modify the second property. Maybe it's worth making read-only properties instead of the whole objects.