perf(ngClass): avoid unnecessary `.data()` accesses, deep-watching and copies

Includes the following commits (see #15246 for details):

- **perf(ngClass): only access the element's `data` once**

- **refactor(ngClass): simplify conditions**

- **refactor(ngClass): move helper functions outside the closure**

- **refactor(ngClass): exit `arrayDifference()` early if an input is empty**

- **perf(ngClass): avoid deep-watching (if possible) and unnecessary copies**

  The cases that should benefit most are:

  1. When using large objects as values (e.g.: `{loaded: $ctrl.data}`).
  2. When using objects/arrays and there are frequent changes.
  3. When there are many `$index` changes (e.g. addition/deletion/reordering in large `ngRepeat`s).

  The differences in operations per digest include:

  1. `Regular expression (when not changed)`
     **Before:** `equals()`
     **After:**  `toClassString()`

  2. `Regular expression (when changed)`
     **Before:** `copy()` + 2 x `arrayClasses()` + `shallowCopy()`
     **After:**  2 x `split()`

  3. `One-time expression (when not changed)`
     **Before:** `equals()`
     **After:**  `toFlatValue()` + `equals()`*

  4. `One-time expression (when changed)`
     **Before:** `copy()` + 2 x `arrayClasses()` + `shallowCopy()`
     **After:**  `copy()`* + `toClassString()`* + 2 x `split()`

  5. `$index modulo changed`
     **Before:** `arrayClasses()`
     **After:**  -

  (*): on flatter structure

  In large based on #14404. Kudos to @drpicox for the initial idea and a big part
  of the implementation.

Closes #14404

Closes #15246