Improve Our Handling of Flow Layout Headers/Footers

[Adlai-Holler]

This is valuable in general, and critical for our support for flow layout headers/footers under IGListKit.

Before this diff:

• Layout talked directly to our delegate about referenceSizeForHeader
• So if you return { 320, 100 }, the layout will make the header 100 tall.
• But flow layout inspector will constrain the header node to { {320, 0}, {320, 100} } when measuring.
• So if your node ends up being 80 tall, then the view it's placed in will be 100 tall but the node will only be 80 tall.
• Our documentation incorrectly states that insetForSectionAtIndex: does nothing. It doesn't do nothing! Flow layout calls this directly and uses it to layout items.

After this diff:

• ASColllectionView will intercept the referenceSizeForHeader call, and return the calculated size for the header node, so they always match up.
• If the delegate continues to implement it, the layout inspector will call referenceSizeForHeaderInSection: to get an exact size constraint for the header.
• Add a new method referenceSizeRangeForHeader which is preferred, so the client doesn't have to know the exact header size.
• Add ASSizeRangeZero and ASSizeRangeUnconstrained constants.