I noticed some older-style code in ClassLoader.c while working on another issue. It's starting to feel like spring where I am, and I'd like to do a little tidying.

Suggested changes:

- Alphabetize includes
- Change instances of 'if (ptr == 0)' to 'if (ptr == NULL)'
- Change instances of 'return 0' to 'return NULL'
- Wrap the bodies of if statements in curly-braces.