Exploding `Nat64` and other fixed-width multibyte types

[ggreif]

Proposed primitive:

func nat64BigEndian(n : Nat64) : (msb : Nat8, Nat8, Nat8, Nat8, Nat8, Nat8, Nat8, lsb : Nat8)

The BigEndian part is necessitated by the representation of Nat8 in the backend. This order is waaaaay more efficient to generate than the reverse.

FYI: @timohanke

[crusso]

Given wasm is little-endian, wouldn't it be most natural and efficient to create a blob in little-endian order? We can also provide big-endian variants, or just get the user to index from the end for big-endian.

If you return a tuple, it will be hard for the user to iterate over the elements...

[ggreif]

If you return a tuple, it will be hard for the user to iterate over the elements...

This is about the primitive which should be as efficient as possible. We can bike-shed about the presentation to base as we want, later.