ENH: Implement core/strings/wrap method
This patch implements the str.wrap function within core/strings.

Example:

```
    >>> s = pd.Series(['line to be wrapped', 'another line to be wrapped'])
    >>> s.str.wrap(12)
    0             line to be\nwrapped
    1    another line\nto be\nwrapped
```

This is a cleaned branch; the original is at #6705.
