ENH: add to_series() method to Index and subclasses GH3275

#3275

```python
In [10]: df=mkdf(10,5,r_idx_nlevels=2)

In [11]: df.index
Out[11]: 
MultiIndex
[R_l0_g0  R_l1_g0, R_l0_g1  R_l1_g1, R_l0_g2  R_l1_g2, R_l0_g3  R_l1_g3, R_l0_g4  R_l1_g4, R_l0_g5  R_l1_g5, R_l0_g6  R_l1_g6, R_l0_g7  R_l1_g7, R_l0_g8  R_l1_g8, R_l0_g9  R_l1_g9]

In [12]: # new method
    ...: df.index.to_series()
Out[12]: 
R0       R1     
R_l0_g0  R_l1_g0    (R_l0_g0, R_l1_g0)
R_l0_g1  R_l1_g1    (R_l0_g1, R_l1_g1)
R_l0_g2  R_l1_g2    (R_l0_g2, R_l1_g2)
R_l0_g3  R_l1_g3    (R_l0_g3, R_l1_g3)
R_l0_g4  R_l1_g4    (R_l0_g4, R_l1_g4)
R_l0_g5  R_l1_g5    (R_l0_g5, R_l1_g5)
R_l0_g6  R_l1_g6    (R_l0_g6, R_l1_g6)
R_l0_g7  R_l1_g7    (R_l0_g7, R_l1_g7)
R_l0_g8  R_l1_g8    (R_l0_g8, R_l1_g8)
R_l0_g9  R_l1_g9    (R_l0_g9, R_l1_g9)
dtype: object

In [13]: # so now this is possible
    ...: df[df.index.to_series().map(lambda x: "4" in x[0])]
Out[13]: 
C0              C_l0_g0 C_l0_g1 C_l0_g2 C_l0_g3 C_l0_g4
R0      R1                                             
R_l0_g4 R_l1_g4    R4C0    R4C1    R4C2    R4C3    R4C4
```