* Revise performance measurement naming and structure
- Revises naming from LayoutSpecGeneration to LayoutSpecComputation
- Adds a struct instead of an NSDictionary to retrieve performance metrics
- Includes ASEnvironmentStatePropagateDown in LayoutSpecComputation measurements
* Revise SumScopeTimer to include enable flag
* Make struct a typedef
* Measure performance for ASCellNode layout
* Address Adlai and Levi's feedback:
* Move to ASDisplayNode level
* Lock around setting/getting measurement options and results
* Record all measurement passes and report times as an array
* Only add relevant entries to the performanceMetrics dictionary
* Rebase
* Store sum and count instead of array
* Rename ScopeTimerDataPoint to ScopeTimerSum
* Address Levi's feedback
* Address Adlai's feedback
