* Use persistent hash map to store _inheritedWidgets

Instead of using a HashMap and copying it down the tree
which leads to quadratic time and space complexity
use a persistent data structure which can amortize
the cost by sharing parts of the structure.

The data shows HAMT based PersistentHashMap to be
5-10x faster for building _inheritedWidgets and
considerably more space effecient (e.g. bringing
amount of memory allocated when constructing
_inheritedWidgets in a tree with 150 InheritedWidget
down to 70Kb from 970Kb).

PersistentHashMap is slower than HashMap for
access: 2-3x in relative terms, but in absolute
terms we are only talking about ~0.2ns slow down
per access and various app benchmarks we run have
have not revealed any significant regressions.