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.