After running source files through the pretty printer, we sanity check them by
running the results through the compiler with the --no-trans flag. Now also
use the --lib flag so that pretty-printer tests don't have to include a main
function.
Add a new src/test/pretty directory to hold just source files for testing the
pretty-printer.
Add a new pp-exact directive. When this directive is followed by a file name
it specifies a file containing the output that the pretty-printer should
generate. When pp-exact is not followed by a filename it says that the file
should pretty-print as written.
