Put the declarations in "data.h", because that header declares the
types cost_t and token_p. Also #include <cgg_cg.h> from "data.h" to
get types c3_p and set_p, and guard <cgg_cg.h> against multiple
inclusion.
*Important:* You must "make clean" after checking out this commit,
because the build had copied the old "assert.h" to several places in
obj/. If you don't "make clean", then the compiler finds the old
"assert.h" before libc <assert.h>, and the build fails because this
commit removes badassertion() in subr.c. After "make clean", the
compiler finds libc <assert.h> and the build succeeds.
