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.

tests/all/main.rs

@@ -8,6 +8,7 @@ mod classes;
 mod closures;
 mod comments;
 mod dependencies;
+mod duplicates;
 mod enums;
 mod import_class;
 mod imports;