The cowbel runtime library is a work in progress and mainly exists to support the test suite. If you actually want to write a real program in cowbel, you're advised to copy the files you want to protect yourself from future changes.
Currently the actual library is so much in flux that the only sensible documentation is in the header files themselves. Formal documentation here will follow once I've written a documentation generator.
Unlike cowbel 0.1, primitive types are now represented by interfaces and are described in