this allows us to reuse the code in wiggle-generate elsewhere, because
a proc-macro=true lib can only export a #[proc_macro] and not ordinary
functions.
In lucet, I will depend on wiggle-generate to define a proc macro that
glues wiggle to the specifics of the runtime.
* atoms in one test unit
* factor out pointers test
* factor structs into separate test unit
* factor out arrays, flags
* finally, separate into strings and ints