Add (naive) type conversion

Something like `convert(::Type{Vector3{Float32}}, ::Vector3{Float64})` is now possible (just like native Julia arrays).

The implementation is naive and uses `ntuple`. I'm guessing it can be improved with `mapBody` somehow, but I was having trouble figuring it out.

This is basically the last bit of functionality needed for me to completely switch over to ImmutableArrays from my homebrew implementation :grinning: