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.

crates/generate/src/lib.rs

@@ -1,5 +1,3 @@
-extern crate proc_macro;
-
 mod config;
 mod funcs;
 mod lifetimes;
@@ -7,20 +5,16 @@ mod module_trait;
 mod names;
 mod types;
 
-use proc_macro::TokenStream;
+use proc_macro2::TokenStream;
 use quote::quote;
-use syn::parse_macro_input;
 
-use config::Config;
-use funcs::define_func;
-use module_trait::define_module_trait;
-use names::Names;
-use types::define_datatype;
-
-#[proc_macro]
-pub fn from_witx(args: TokenStream) -> TokenStream {
-    let config = parse_macro_input!(args as Config);
+pub use config::Config;
+pub use funcs::define_func;
+pub use module_trait::define_module_trait;
+pub use names::Names;
+pub use types::define_datatype;
 
+pub fn generate(config: Config) -> TokenStream {
     let doc = witx::load(&config.witx.paths).expect("loading witx");
 
     let names = Names::new(config); // TODO parse the names from the invocation of the macro, or from a file?
@@ -43,10 +37,10 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
         )
     });
 
-    TokenStream::from(quote!(
+    quote!(
         mod types {
             #(#types)*
         }
         #(#modules)*
-    ))
+    )
 }