added getSize function with dirty checking

Summary:
Adds check for smaller clipSize than context size

size defaults to 2d undefined array

Handles 0 case for index

Added getAbsolutePosition

Added checks for null size and subset checks

Added trackSize option, refactored to cumulative size

Constant time lookup of size

ViewSequence trackSize option

Test Plan: Scrollview demo

Reviewers: mike, daniel

Subscribers: larry

Differential Revision: https://phabricator.famo.us/D71
Removed size cleanup