Implement `position: absolute` for non-replaced elements.

+ Re-implement fixed positioning using the absolute positioning code.
+ Add reftests for absolute positioning and fixed positioning.
+ Refactor assign_widths in BlockFlow to isolate the calculation of
widths and margins.
+ Pass down details of the Containing Block for absolute and fixed flows
during layout. Use it to calculate the static position of absolute flows.
+ Defer calculation of absolute flow dimensions till we build the
display list.
Implement mini-traversal for absolute flow assign-height.

This only traverses absolute flows, nothing else.

+ Also, a separate mini-traversal for store overflow.

+ Add basic fields for absolute descendants and CB to BaseFlow.

+ Bubble up absolute descendants in the Containing Block during Flow Construction.

+ Set Rawlink to the CB in absolute descendants.

+ store_overflow() now uses absolute descendants' overflows too.

+ Add reftests for `position: absolute` static y position.

+ Add reftests for overflow (they all fail now).