Make arrays able to nest

Prior to this commit, the following type was considered invalid: [[float]]

Of course, sometimes it is helpful to have arrays within arrays, and as such, this patch allows for nested arrays. To do this, the array types in Primitive have all been converted to a Primitive::Array type, that holds a Box<> to another Primitive. There is - of course - a slight performance penalty to be expected with moving to using another Box for arrays, however after having run the benchmarks, the difference appears to be negligible.