Add consV, snocV for building NonEmptyVector

[AlistairB]

from an A and a Vector.

Also, cons and snoc can be defined in terms of consV and snocV now.

Closes #7

An alternative way to think about (and name) these would be as construction functions ie. mkNonEmptyVector :: a -> Vector a -> NonEmptyVector a. I suppose once you consV you then have a NonEmptyVector, so you wouldn't chain multiple consV.

[emilypi]

LGTM - thanks for doing the docs too. I'll merge on green and do a minor point release. To your point: i kind of like consV and snocV as a descriptor as opposed to the wordier version.