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/cli-support/src/lib.rs

@@ -198,6 +198,7 @@ impl Bindgen {
                 exposed_globals: Default::default(),
                 required_internal_exports: Default::default(),
                 imported_names: Default::default(),
+                imported_identifiers: Default::default(),
                 exported_classes: Default::default(),
                 config: &self,
                 module: &mut module,