Support ClassLoader#getResources with multiple class path elements

When there are multiple .jar files in the class path, ```ClassLoader.getResources("META-INF/MANIFEST.MF");``` is supposed to return an enumeration of all the matching class path elements, not just the first match.

This is important in particular for [Sezpoz](https://sezpoz.java.net/) which parses annotations at compile time and stores the results in ```META-INF/annotations/``` in the corresponding _.jar_ files.

Note: I am not quite sure about the _opaque_ parameter in https://github.com/dscho/avian/commit/49303ad4d64dabd5e3e5866b60de1a4d3779f6e1 -- I could imagine that a better way would be to edit ```src/types.def``` but all my attempts resulted in a segmentation fault when the generator runs...