generator: open Unix module by default.

generator: open Unix module by default.

Add:

  open Unix

at the top of the generator, which means that we don't need to
prefix any 'Unix.foo' symbols (we can just use 'foo' instead).

Unfortunately the Unix module shadows one symbol in Pervasives
(the Pervasives module is opened by default in OCaml code).  That
symbol is 'stdout'.  So we replace this with 'Pervasives.stdout'
in two places.  Still a net reduction in code size.