Move example program generation to its own file

This makes it possible to `(load "compiler.scm")` without getting some
semi-random assembler printed at startup

scm/inc/Makefile

asm: scheme.s
	cat scheme.s
scheme.s: compiler.scm
	petite --script compiler.scm > $@
scheme.s: compiler.scm program.scm
	petite --script program.scm > $@
integers.s: integers.c
	gcc -O3 --omit-frame-pointer -S $<

scm/inc/compiler.scm

  (emit-expr x)
  (emit "ret"))
(compile-program '(null? ()))

scm/inc/program.scm

(load "compiler.scm")
(compile-program '(null? 0))