Generated class filenames are too long for some systems

Some systems, like windows and ecrypted filesystems on linux, have a maximum path length of around 255. Due to scalac's autogenerated class names, it's very possible to read this limit. We should use the -Xmax-classfile-name option to limit the length of these autogenerated names. Note that our longest dirnames are about 100 characters, so if we can limit the length of filenames to less than 100, that gives users about 50 characters for a root directory path, which isn't totally unreasonable.