Conditionalize wasm_bindgen to fix compile error on wasm32-wasi

From investigation for #115, now three targets:

- wasm32-unknown-unknown: build with `wasm-pack build`, uses
the #[wasm_bindgen] directive on main()

- wasm32-wasi: build with `cargo +nightly build --target wasm32-wasi`,
requires normal main()

- native targets: same as wasm32-wasi

src/main.rs

@@ -44,7 +44,6 @@ pub mod model;
 pub mod entity;
 
 use cfg_if::cfg_if;
-use wasm_bindgen::prelude::*;
 use std::sync::{Arc, RwLock, Mutex};
 use std::rc::Rc;
 use std::marker::PhantomData;
@@ -190,8 +189,19 @@ cfg_if! {
     }
 }
 
-#[wasm_bindgen]
-pub fn main() {
+cfg_if! {
+    if #[cfg(target_os = "unknown")] {
+        use wasm_bindgen::prelude::*;
+
+        #[wasm_bindgen]
+        pub fn main() { main2(); }
+    } else {
+        #[inline]
+        pub fn main() { main2(); }
+    }
+}
+
+fn main2() {
     let opt = Opt::from_args();
 
     set_panic_hook();