[snshn]

The WHATWG MIME type parser used for data URLs is now public as
dataurl::mime::MimeType (parse, essence, main_type, subtype, parameters,
charset, Display, FromStr), e.g. for parsing Content-Type headers.
DataUrl::mime_type() returns a MimeType, and DataUrl::set_mime_type()
sets the media type with all of its parameters from one.

How data gets serialized is now controlled by two settings:

• DataEncoding (Percent, Base64, or Shortest, which picks whichever
  results in a shorter data URL), replacing is_base64_encoded() and
  set_is_base64_encoded()
• Escaping (Full, the default, safe anywhere; or Minimal, which only
  escapes what would break double-quoted HTML attributes, CSS strings,
  and STYLE/SCRIPT elements), which keeps text formats like SVG short

Percent-encoding now also escapes "&", and trailing spaces always get
escaped, since URL parsers strip them.