Teach bstr/ustr to provide repr of themselves: it goes as b(...) and
u(...) where u stands for human-readable repr of contained data.
Human-readable means that non-ascii printable unicode characters are
shown as-is instead of escaping them, for example:

    >>> x = u'αβγ'
    >>> x
    'αβγ'
    >>> y = b(x)
    >>> y
    b('αβγ')				<-- NOTE not b(b'\xce\xb1\xce\xb2\xce\xb3')
    >>> x.encode('utf-8')
    b'\xce\xb1\xce\xb2\xce\xb3'