add lens-if, lens-cond, and lens-match

[AlexKnauth]

> (define if-lens (lens-if list? first-lens (vector-ref-lens 0)))
> (lens-view if-lens '(1 2 3))
1
> (lens-view if-lens '#(1 2 3))
1
> (lens-set if-lens '(1 2 3) 'a)
'(a 2 3)
> (lens-set if-lens '#(1 2 3) 'a)
'#(a 2 3)

[jackfirth]

Very cool