* 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.
