ENH: Allow fast comparisons of Index views, similar to 'is' checks.

[jtratner]

Related: #4039 (where this started to become an issue), #4202, #4830, #4628, #4718, #4628.

Very simple: both indices get an _id property that can be compared with is and is reset every time an object is created, so it only stays set when set explicitly (like in view(), but not in copy()). Adds an is_ method that wraps this for external use and makes it so you don't need hasattr checks, etc.

>>> ind = Index(range(10))
>>> ind.is_(ind)
True
>>> ind.is_(Index(range(10)))
False
>>> ind.is_(ind.view().view().view().view())
True
>>> ind.is_(ind.copy())
False
>>> ind.is_(ind.copy(deep=False))
False
>>> ind.is_(ind[:])
False
>>> ind.is_(ind.view(np.ndarray).view(Index))
False
>>> ind.is_(np.array(range(10)))
False
>>> ind.is_(ind.view().base) # quasi-implementation dependent
True
>>> ind2 = ind.view()
>>> ind2.name = 'bob'
>>> ind.is_(ind2)
True
>>> ind2.is_(ind)
True
>>> # doesn't matter if Indices are *actually* views of underlying data, e.g.
>>> ind.is_(Index(ind.values))
False
>>> arr = np.array(range(1, 11))
>>> ind1 = Index(arr, copy=False)
>>> ind2 = Index(arr, copy=False)
>>> ind1.is_(ind2)
False

MultiIndex behavior (same behavior for set labels and set levels)

>>> mi = MultiIndex.from_tuples(zip(range(10), range(10)))
>>> mi.is_(mi)
True
>>> mi.is_(mi.view())
True
>>> mi.is_(mi.view().view().view().view())
True
>>> mi2 = mi.view()
>>> mi2.names = ["A", "B"]
>>> mi2.is_(mi)
True
>>> mi.is_(mi2)
True
>>> mi.is_(mi.set_names(["C", "D"]))
True
>>> mi3 = mi2.set_levels([range(10), range(10)])
>>> mi3.is_(mi2)
False
>>> mi4 = mi3.view()
>>> mi4.set_levels([[1 for _ in range(10)], range(10)], inplace=True)
>>> mi4.is_(mi3)
False
>>> mi5 = mi.view()
>>> mi5.set_levels(mi5.levels, inplace=True)
>>> mi5.is_(mi)
False

Could make it easier to resolve #4013.