Fix caching for flexbox columns

[nicoburns]

Objective

Fixes a perf issue due to cache-thrashing that occurs with Flexbox columns.

Changes made

Increased caches slots per node from 7 to 9, and updated slot-assingment algorithm to make use of those extra slots. specifically, MinContent and MaxContent/Definite sizes are now cached separately in both axis.

Context

Found when investigating the following issues:

• measure_func is called too many times when minWidth/maxWidth is involved #555
• Yoga does not respect automatic minimum size for flexbox items facebook/yoga#1409

Potential future improvements

• Optimise cache size by taking advantage of invariants encoded by the cache slot assignment algorithm. For example, slot 8 is only used if known_dimensions are (None, None) and available_space is (MinContent, MinContent). So storing the value of those inputs as cache keys is unnecessary for that slot.
• Improve benchmarks. It has become apparent that there are a lot of cases our current benchmarks don't cover: columns, all-auto sized nodes, flex wrapping, measure functions (in particular: measure functions that return non-constant sizes and slow measure functions), etc.