io: add chdir

Adding the chdir function, that will change the current directory of any
running chapel program.

This includes two tests: one that tries to chdir to a directory that doesn't
exist, and other that calls chdir, and then writes a file to the current
directory (a new one).

Please let me know if I need to improve anything!
Yeah, this was my "automatic mode", my text editor highlights these whitespaces and I removed then with a command. I will remove the change related to them before merging. Thanks for reviewing!