The reading of a file by the ranges of its bytes that a reader needs,
fetched in rounds within limits, for the readers of views.

A reader of a view gives the ranges that it lacks, and read! fetches
them and reads again, until the reader has what it needs. The bytes come
from a fetch function: an http/fetcher for a file on the web, or a
fetcher of a local file or a Blob.

Views of files whose bytes come in parts, for the readers of media files
and images, on both platforms.

A view holds the parts of a file that are there, each at its offset. A
reader asks the view for the ranges that it needs before it reads them.
A range that the view doesn't hold throws an exception that holds the
ranges, so that the caller can fetch them and read again. A range past
the end of the file throws too. The fetching is in util.bytes.ranges.