[PSyIR] Arrays of scalars should be arrays of ScalarType

[arporter]

Currently an array of Scalars is represented by an ArrayType and the type and precision of an element are given by the intrinsic and precision properties of that class, respectively. However, this is inconsistent with other sorts of an array - an array of Structures for instance has the intrinsic as the StructureType.

We therefore need to make arrays of scalars consistent with other sorts of arrays. For clarity, we will also rename ArrayType.intrinsic to be ArrayType.elemental_type and (I think) do away with precision altogether since that is captured in ScalarType.