Support spread elements in const collections.

[munificent]

The discussion on #63 convinced me this is desirable and feasible, so I updated the proposal.

How does this look?

[lrhn]

I'm still split. It's definitely feasible, but it extends the usefulness of constant list/maps in just one direction ,without doing it generally. Why can't we do length on such constant maps and lists, or use the index operator with a constant operand? Our limits are arbitrary, and this just exposes that (more).

On the other hand, spreading only allows you to create larger objects, not to inspect the contents of the individual objects, so there is still a kind of reasoning to the limitations.

Leaning LGTM. Let's do it.

[munificent]

It's definitely feasible, but it extends the usefulness of constant list/maps in just one direction ,without doing it generally. Why can't we do length on such constant maps and lists, or use the index operator with a constant operand?

That's true of any basically any proposal that touches const. Adding any territory on either the const side or non-const side increases the length of the boundary between them, which increasingly approaches infinite length. :)