[BUGFIX release] Simplify get and computeds

[pzuraq]

This refactor simplifies get and computeds in the following ways:

• Removes several brand checks that were unnecessary from get:
  • isEmberArray: Ember Arrays that are not native arrays will
    automatically be tracked through proper usage.
  • isProxy: This was only necessary for conditionals, and updates in
    the VM allow us to autotrack that instead.
• Uses tagFor instead of tagForProperty in many places including
  get. This allows us to avoid the brand check for
  CUSTOM_PROPERTY_TAG, which is really only necessary for chain tags.
• Updates everywhere that looks up tags multiple times on the same
  object to use a shared tagMeta so we don't lookup that map multiple
  times.
• Moves computed revision and value caches onto Meta, so we're looking
  up fewer maps.
• Creates a new AutoComputed descriptor for computed properties that
  use autotracking, so we can simplify the get logic for standard CPs.

[pzuraq]

Benchmark results against current master of Ember and Glimmer VM:
artifact-53.pdf