First and last via initial/rest

[megawac]

These cases are probably not very important anyway if #1758 is merged as it can be handled by via negative indexes to slice.

[akre54]

The perf diff is negligible and I dig the reusability. We can cross the _.slice stream when we come to it.

[jdalton]

I donno if it matters but you were perf testing the wrong thing since the _.rest and _.initial use only kick in when the methods are passed an n value.

[akre54]

Oops, updated. Still isn't horrible though.