*     process(part.headers, part.body)
 *   }
 * }
 * ```
 *
 * Note that [nextPart] will skip any unprocessed data from the preceding part. If the preceding
 * part is particularly large or if the underlying source is particularly slow, the [nextPart] call
 * may be slow!
 *
 * Closing a part **does not** close this multipart reader; callers must explicitly close this with
 * [close].
 *