canonicalizing values (like "interning" or "hash-consing").

Any value of comparable type may be canonicalized with the new
`Make[T]` function, which produces a reference to a canonical copy of
the value in the form of a `Handle[T]`.
Two `Handle[T]` are equal if and only if the values used to produce the
handles are equal, allowing programs to deduplicate values and reduce
their memory footprint.

pkg mime/multipart, type FileHeader struct, Filename string
pkg mime/multipart, type FileHeader struct, Header textproto.MIMEHeader
pkg mime/multipart, type Form struct
pkg mime/multipart, type Form struct, File map[string][]*FileHeader
pkg mime/multipart, type Form struct, Value map[string][]string
pkg mime/multipart, type Part struct
pkg mime/multipart, type Part struct, Header textproto.MIMEHeader

modification, are permitted provided that the following conditions are
met:

   * Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following disclaimer
in the documentation and/or other materials provided with the
distribution.
   * Neither the name of Google Inc. nor the names of its