Add isomorphism between positive integers and lists of digits

[jackfirth]

Should accept a base and return an isomoprhism lens

> (lens-view (positive-integer->digits-lens 10) 123) ; base 10
'(1 2 3)
> (lens-view (positive-integer->digits-lens 2) 123) ; base 2
'(1 1 1 1 1 1 1)