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.

src/lib.rs

@@ -1,5 +1,10 @@
-/*
-pub mod wasi {
-    generate::from_witx!("crates/WASI/phases/snapshot/witx/wasi_snapshot_preview1.witx");
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use syn::parse_macro_input;
+
+#[proc_macro]
+pub fn from_witx(args: TokenStream) -> TokenStream {
+    let config = parse_macro_input!(args as wiggle_generate::Config);
+    TokenStream::from(wiggle_generate::generate(config))
 }
-*/