Remove packed numbers (Num2 / Num4)

[irh]

As much as I like the idea of having the Num2 and Num4 types in Koto, there are good reasons for removing them.

• They're not mutable, so any operations on them have to be assigned, which is fiddly in practice.
• They add complexity to the language, making Koto harder to learn.
• Their presence adds complexity for library authors (more than one standard number type to accept as function arguments).
• Having standard packed number types means their standard library modules need to cover too many use cases.
  • e.g. should num4 be a color type or a 3d or 4d vector? Or a rect?
• Libraries/apps will generally add wrappers for their own internal geometry or color ops.

In the end, I think their value is limited compared to the cost of including them, so they should be removed as soon as possible to avoid breaking too many existing programs.