Simplify and compact transitions storage

Simplify and compact transitions storage

Simple transitions are now stored in a map's "transitions" field (as a WeakCell wrapping the target map); full TransitionArrays are used when that's not sufficient.
To encapsulate these storage format implementation details, functions for manipulating and querying transitions have been refactored to be static functions on the TransitionArray class, and take maps as inputs.

Review URL: https://codereview.chromium.org/980573002

Cr-Commit-Position: refs/heads/master@{#27029}