ENH: Add `sorted_keys` kwarg to `np.searchsorted`

If both the needle and haystack are sorted, instead of a binary search,
Which is O(n log m), use linear search, which is O(n + m). The speed-up
depends greatly on the size of the inputs, but for sufficiently large
equally sized arrays, 5x faster is a typical value:

```
In [57]: a = np.sort(np.random.rand(1000))

In [58]: b = np.sort(np.random.rand(1000))

In [59]: %timeit a.searchsorted(b)
10000 loops, best of 3: 31.8 us per loop

In [60]: %timeit a.searchsorted(b, sorted_keys=True)
100000 loops, best of 3: 6.69 us per loop
```