Add promise flattening and lazy boxing

[robertdp]

For #2 and #3

Short summary:

• the FFI types become more polymorphic (and unsafe)
• a new Flatten class models how promises automatically flatten
• LazyPromise now has all values wrapped in a new Box container
• added LazyPromise.new

[thomashoneyman]

Thanks for taking this on, @robertdp! I'll have a little time to review this soon, though I would also appreciate a look from @natefaubion, who originally raised this issue.

[natefaubion]

I think this looks really good. The only question I had was whether we need to export resolve from Web.Promise with a Flatten constraint.

[robertdp]

Ah yes, good catch. The changes to the FFI types make it too permissive to export as-is.

I'll push a fix shortly.