Rename namespace to js_namespace

Along the way remove the namespace in Rust as this ended up causing too many
problems, alas! The `js_namespace` attribute now almost exclusively modifies the
JS bindings, hence the "js" in the name now.

crates/wasm-bindgen-shared/src/lib.rs

@@ -20,7 +20,7 @@ pub struct Program {
 #[derive(Deserialize)]
 pub struct Import {
     pub module: Option<String>,
-    pub namespace: Option<String>,
+    pub js_namespace: Option<String>,
     pub kind: ImportKind,
 }