And a corresponding `created` field to the voucher model object which
propagates to the web view.

Extraction order does not need to equal insertion order.

Turns out, it did already.  Make the use order explicit in the model, anyway.

Thanks, Hypothesis.

Probably useful for implementation later.
Useful for test suite right now.

Helpful for the tests

This helps the client a lot with lease operations

The native st_size-based approach to size estimation is broken because mutable
and immutable layouts have a different amount of overhead.  So start peeking
inside the share files.

Also, new_length must be respected now because when it is used the part of the
share file we peek at changes.  Also, new_length can *only* truncate so stop
treating it as if it can extend.

Do it another way.

and without new_length

Instead of assuming all shares are the same size here, pass in all the sizes.
The protocol doesn't allow immutable shares in a storage index to have
differences in size but it does allow mutable shares in a slot to differ this
way.