wasmtime-wiggle-macro: re-use Names functionality

rather than try to duplicate it in utils

crates/wasi/src/lib.rs

@@ -8,6 +8,7 @@ pub use wasi_common::{WasiCtx, WasiCtxBuilder};
 // with all the various WASI exports.
 wasmtime_wiggle::define_struct_for_wiggle!({
     witx: ["../wasi-common/WASI/phases/snapshot/witx/wasi_snapshot_preview1.witx"],
+    ctx: WasiCtx,
 });
 
 pub fn is_wasi_module(name: &str) -> bool {