smoke test for the ir-driven transpiler: every translation
unit starts with the core preamble, optionally declares types
(forward + full def in topological order), then forward-decls
all functions, then bodies, then the runtime trailer when a
`main` is present. `main` is renamed to `catskill_main` so the
runtime can wrap it.

<<<

main = fun () {
}

>>>

#include "core.c"
void catskill_main(void);
void catskill_main(void) {
}
#include "runtime.c"