add reverse-lens and last-lens

[AlexKnauth]

> (lens-view reverse-lens '(1 2 3))
'(3 2 1)
> (lens-transform reverse-lens '(1 2 3) (cons 4 _))
'(1 2 3 4)
> (lens-view last-lens '(1 2 3))
3
> (lens-set last-lens '(1 2 3) 'a)
'(1 2 a)