Support dynamic spreads.

[back2dos]

Especially when working with React passing dynamic props is desired, which is quite different from tink.Anon.merge that underpins hxx spreads. Syntax would be {...untyped <expr>}. Have to figure out what exactly to do regarding type checking. Probably should support a way to disallow it for specific tags (e.g. coconut views).

[kevinresol]

How about (top-down) typing the attributes without the dynamic spread first. Then just reuse that type for the post-spread object?

function dynamicSpread<T>(staticAttrs:T, dynamicAttrs:Any):T;