get_type(): Use a compiled regular expression, which can be shared.

_get_params_preserve(): A helper method which extracts the header's
    parameter list preserving value quoting.  I'm not sure that this
    needs to be a public method.  It's necessary because we want
    get_param() and friends to return the unquoted parameter value,
    however we want the quote-preserved form for set_boundary().

get_params(), get_param(), set_boundary(): Implement in terms of
    _get_params_preserve().

walk(): Yield ourself first, then recurse over our subparts (if any).