ENH: Add 'is_' method to Index for identity checks

Adds a method that can quickly tell whether something is a view or not
(in the way that actually matters).

CLN: change 'is' check to 'is_' check in 'equals()'
Merge pull request #4909 from jtratner/add-is_-method-to-index

ENH: Add 'is_' method to Index for identity checks