Fix some situations with duplicate imports (#589)

* Fix importing the same identifier from two modules

This needed a fix in two locations:

* First the generated descriptor function needed its hash to include the module
  that the import came from in order to generate unique descriptor functions.
* Second the generation of the JS shim needed to handle duplicate identifiers in
  a more uniform fashion, ensuring that imported names didn't clash.

* Fix importing the same name in two modules

Previously two descriptor functions with duplicate symbols were emitted, and now
only one function is emitted by using a global table to keep track of state
across macro invocations.

crates/backend/src/lib.rs

@@ -2,6 +2,8 @@
 #![cfg_attr(feature = "extra-traits", deny(missing_debug_implementations))]
 #![doc(html_root_url = "https://docs.rs/wasm-bindgen-backend/0.2")]
 
+#[macro_use]
+extern crate lazy_static;
 #[macro_use]
 extern crate log;
 extern crate proc_macro2;