Prototype for all values

[71104]

All Lambda values must have a prototype, not just objects and something else. The prototype will be empty for some values, namely undefined and null.

The behavior of some let statements will change. For example, the following statement:

let x = 5 in
let x.x = 6 in
x

currently yields an object with only one field; the name of the field is x and the value is 6.

After this feature is implemented, the above will yield a number (5) whose context is augmented with an extra field x whose value is 6.

[71104]

We should also consider dropping "objects" completely in favor of simply extending the prototype of undefined.

It's completely reasonable that the following yield undefined:

let x.a = 5 in x

But this poses the problem of how to distinguish undefined from empty objects when marshalling values to JavaScript APIs.

[71104]

c8aa3e4 provides a partial fix. I still want to remove "objects" in favor of extending the prototype of undefined. This will simplify the value domain.