Make LazyMap and AbstractVector (#253)

Author: blegat

src/lazy_iterators.jl

@@ -32,7 +32,7 @@ Iterator over the elements of `data` mapped by `f`. This is similar to
 `Base.Generator(f, data)` except that the `eltype` of a `LazyMap` is given at
 construction while the `eltype` of `Base.Generator(f, data)` is `Any`.
 """
-struct LazyMap{T,VT,F}
+struct LazyMap{T,VT,F} <: AbstractVector{T}
     f::F
     data::VT
 end