Rename static to namespace

This commit renames the `static` attribute to `namespace` and simultaneously
reduces and expands the scope. The `namespace` attribute can now be applied to
all imports in addition to functions, and it no longer recognizes full typed
paths but rather just a bare identifier. The `namespace` attribute will generate
a Rust namespace to invoke the item through if one doesn't already exist (aka
bindign a type).

examples/console_log/src/lib.rs

@@ -6,9 +6,7 @@ use wasm_bindgen::prelude::*;
 
 #[wasm_bindgen]
 extern {
-    type console;
-
-    #[wasm_bindgen(static = console)]
+    #[wasm_bindgen(namespace = console)]
     fn log(s: &str);
 }