make wiggle-generate ordinary lib, and wiggle the proc-macro lib

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.

tests/ints.rs

@@ -3,7 +3,7 @@ use std::convert::TryFrom;
 use wiggle_runtime::GuestError;
 use wiggle_test::{impl_errno, HostMemory, MemArea, WasiCtx};
 
-wiggle_generate::from_witx!({
+wiggle::from_witx!({
     witx: ["tests/ints.witx"],
     ctx: WasiCtx,
 });