[BUGFIX release] Simplify mixin application

Simplifies mixin application in a number of ways:

- Ensures that we only access `meta` once for a given object when
  applying mixins
- Ensures that we only `peekDescriptors` once for a given descriptor
- Minimizes the number of if/else branches and defaulting in general
- Breaks apart `defineProperty` so that we can do less work per
  definition when mixing in mixins, since we know more about what
  possible operations will occur.
- Removes extra brand checks from `defineProperty` (Array.isArray) so
  we don't penalize every defineProperty for doing that.
- Only revalidate observers once per mixin application.
- Only brand check `didDefineProperty` once per mixin application.
- Replace `for..in` loops with `Object.keys` since we only care about
  own properties.
- Only loop once in `extractAccessors`.
- Combine observer and listener meta into a single object so we only
  need to do one lookup. In most cases, this is undefined in modern
  applications, so no extra memory cost.
- Simplify CP descriptor property lookups (remove duplicate functions).

(cherry picked from commit f77f5a4)

packages/@ember/-internals/metal/index.ts

@@ -38,13 +38,11 @@ export {
   DecoratorPropertyDescriptor,
   isElementDescriptor,
   nativeDescDecorator,
-} from './lib/decorator';
-export {
   descriptorForDecorator,
   descriptorForProperty,
   isClassicDecorator,
   setClassicDecorator,
-} from './lib/descriptor_map';
+} from './lib/decorator';
 export { default as libraries, Libraries } from './lib/libraries';
 export { default as getProperties } from './lib/get_properties';
 export { default as setProperties } from './lib/set_properties';

packages/@ember/-internals/metal/lib/alias.ts

@@ -16,10 +16,10 @@ import { finishLazyChains, getChainTagsForKey } from './chain-tags';
 import {
   ComputedDescriptor,
   Decorator,
+  descriptorForDecorator,
   isElementDescriptor,
   makeComputedDecorator,
 } from './decorator';
-import { descriptorForDecorator } from './descriptor_map';
 import { defineProperty } from './properties';
 import { get } from './property_get';
 import { set } from './property_set';