Implementing Series.droplevel

[itholic]

This should close #1615

>>> kser = ks.Series(
...     [1, 2, 3],
...     index=pd.MultiIndex.from_tuples(
...         [("x", "a"), ("x", "b"), ("y", "c")], names=["level_1", "level_2"]
...     ),
... )
>>> kser
level_1  level_2
x        a          1
         b          2
y        c          3
Name: 0, dtype: int64

Removing specific index level by level

>>> kser.droplevel(0)
level_2
a    1
b    2
c    3
Name: 0, dtype: int64

Removing specific index level by name

>>> kser.droplevel("level_2")
level_1
x    1
x    2
y    3
Name: 0, dtype: int64

[HyukjinKwon]

Let's combine and address the leftover issues here @itholic

[itholic]

@HyukjinKwon I got it! Thanks for the notice

[itholic]

I think this PR ready to get review.

[itholic]

Thanks, @ueshin. I'll manage things that you pointed out.

[ueshin]

LGTM.

[ueshin]

Thanks! merging.