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!
You should surround this with a check to make sure the directory doesn't already exist.  While I don't think the error will be put in the output file, it'll avoid excess lines in our nightly testing output.