refactor(developer): split kmxplus builder functions into separate files

This refactor work simply splits the existing functions into separate
files to prevent kmx-plus-builder.ts getting too big.

The functions now have no side-effects. Some of them mutate input
parameters, but they are fairly self-contained, which should make
unit tests simpler. Note that `alloc_string` is used extensively, but
works on the input parameter `sect_strs`.