refactor(developer): each section compiler is a pure function

Well, almost. There are still callback side-effects for messages. These
may in future be returned as an array as well, eliminating the need for
a message callback.

But restructures each section compiler so that it is self-contained and
responsible only for writing its own section. Also tightens up the types
for the sections constants.

Last, but not least, moves responsibility for writing the output .kmx
out of compiler.ts and into its callers -- removing one dependency from
the compiler. Reading input files still needs to be done in the
compiler, as there may be multiple input files with `<import>` that we
cannot know about until we start parsing xml.