pp: fix formatting for simple slices

Calling PP on a simple slice (such as a []byte) makes for some very ugly output eg:

pp.PP([]byte("Hello, World!") =>
[
    72,
    101,
    108,
    108,
    111,
    44,
    32,
    87,
    111,
    114,
    108,
    100,
    33,
]
