The hex_lower and hex_upper algorithms could have options for putting in
automatic line breaks, like base64.

Percent encoding doc has a warning that it's not binary safe.  I should
either remove that warning or address the problem (if it is a problem).

Make it possible to do percent encoding as application/x-www-form-urlencoded:
    http://www.w3.org/TR/html4/interact/forms.html#h-17.13.4.1

base16 and base32: http://tools.ietf.org/html/rfc4648
