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/handles.rs

@@ -4,7 +4,7 @@ use wiggle_test::{impl_errno, HostMemory, MemArea, WasiCtx};
 
 const FD_VAL: u32 = 123;
 
-wiggle_generate::from_witx!({
+wiggle::from_witx!({
     witx: ["tests/handles.witx"],
     ctx: WasiCtx,
 });