Spread arrays #237

Example from ES6

[1, 2, ...items, 3, ...moreItems]

Type checking

The type of the array is the best common type of the individual elements and the element types of each of the "spread" elements.

Codegen

[1, 2, ...items, 3, ...moreItems]

becomes:

[1,2].concat(items, [3], moreItems)

If the array starts with a spread, you can concat off the spread:

[...items, 1, 2, ...moreItems]

becomes:

items.concat([1, 2], moreItems)

Questions

What if it's 'any'?  We don't know, so we have to codegen a slice. One possible issue is that .concat doesn't take array-like, it takes arrays only.  This won't be a problem for spread arguments because .apply should take array-likes.