I'm done with this module, how do I get rid of all the object files and other cruft from the source tree?

You have three options, make clean, make distclean or removing the directory. Using make clean will remove all the object files, built executables, and miscellaneous cruft, it's nice and safe. The next option, make distclean should remove everything that wasn't in the original tarball. If you do this, you will not be able to uninstall the module. The same goes for removing the source directory. Basically, make clean is good for general use, the others should be considered drastic measures.