Optional entry support for map and message literal construction

[TristonianJones]

Introduces parse, type-check, and runtime support for optional entry construction on map and message literals, including:

• {?key: a.?b} - create a new map with a dynamic key if the field b is set on object a
• Msg{?item: list[?0]} - create a new Msg with the item field set if the list contains at least one element.

If all of the fields in the object are empty, then using the optional.ofNonZeroValue() call with the result makes it possible to return optional.none() if the value is empty. This is useful for setting optional fields where the value is a map or message type

Msg{
   ?map_value: optional.ofNonZeroValue({
       ?'key': a.?b
   })
}