Add list-files() to filelib.

The list-files() function allows Pyret programs to get a list of files
in a given directory. It can be used like:

  import filelib as FL
  print(FL.list-files("."))
Merge remote-tracking branch 'origin/master' into type-check
Add `type-check-tests' make target.

The type-checking tests can now be run automatically. All files in
tests/type-check/good should result in ok(code), and all files in
tests/type-check/bad should result in err(errs).