Slightly refine comment
Rearrange some code order

Working on the number code and copying from the unit material, it
becomes obvious that the order here is a bit odd!
Rename "unit/inter-unit-product" to "unit/product"

Repeating 'unit' here seems unnecessarily long: the only question is
if the new name is clear enough.
Some initial structures for the number parsing/formatting source

Really just copy-paste from the unit set up.