refactor(equalsFunction): divide internal implemenation

[m-amr]

extract complex logic to named methods
fixes #13957

[gkalpak]

I don't feel refactoring everything out into functions makes it much more clear (if at all).
Also, note this may have perf implications, considering equals is used in some critical paths.

[lgalfaso]

Hi, thanks for the PR! Unfortunately I fully agree with @gkalpak and I find that this change does not make angular.equals any simpler to understand.

[m-amr]

@gkalpak @lgalfaso I respect your decision
but the equals was a very complex function, with a lot of cases but now after
refactoring it just.

  function equals(o1, o2) {
      if (areBothEqualValue(o1, o2)) return true;
      if (isAnyOneHasValueEqualNull(o1, o2)) return false;
      if (areBothValuesEqualNaN(o1, o2)) return true;
      if (areBothObjectsEqual(o1, o2)) return true;
      return false;
  }

I didn't change any logic or and all cases are checked in the same order as before.
I refactored it by extracting functions with a meaningful names. to make it tell the story to reader
now it clear that we are doing the following steps

1 - check if their values are equal using areBothEqualValue(o1, o2) function
2 - check if one of them is null using AnyOneHasValueEqualNull(o1, o2) function
3 - check if their values equal NAN using areBothValuesEqualNaN(o1, o2) function
4 - check if two objects are using equal areBothObjectsEqual(o1, o2) function

I think using a meaningful function names make it easier to understand and better than adding
inline code comments that have the risk to be not updated with the code.
based on http://www.amazon.com/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882/ref=sr_1_1?ie=UTF8&qid=1454769207&sr=8-1&keywords=clean+code
chapter 4 (Comments )

I think it now easier to understand and easier to change too. and each function have just one responsibility is compare a single case. i hope you check it again.
it is a very big function and you need to follow all the paths to understand what is happening and has a lot of nested conditions which make it harder to change.

function equals(o1, o2) {
  if (o1 === o2) return true;
  if (o1 === null || o2 === null) return false;
  if (o1 !== o1 && o2 !== o2) return true; // NaN === NaN
  var t1 = typeof o1, t2 = typeof o2, length, key, keySet;
  if (t1 == t2 && t1 == 'object') {
    if (isArray(o1)) {
      if (!isArray(o2)) return false;
      if ((length = o1.length) == o2.length) {
        for (key = 0; key < length; key++) {
          if (!equals(o1[key], o2[key])) return false;
        }
        return true;
      }
    } else if (isDate(o1)) {
      if (!isDate(o2)) return false;
      return equals(o1.getTime(), o2.getTime());
    } else if (isRegExp(o1)) {
      if (!isRegExp(o2)) return false;
      return o1.toString() == o2.toString();
    } else {
      if (isScope(o1) || isScope(o2) || isWindow(o1) || isWindow(o2) ||
        isArray(o2) || isDate(o2) || isRegExp(o2)) return false;
      keySet = createMap();
      for (key in o1) {
        if (key.charAt(0) === '$' || isFunction(o1[key])) continue;
        if (!equals(o1[key], o2[key])) return false;
        keySet[key] = true;
      }
      for (key in o2) {
        if (!(key in keySet) &&
            key.charAt(0) !== '$' &&
            isDefined(o2[key]) &&
            !isFunction(o2[key])) return false;
      }
      return true;
    }
  }
  return false;
}