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.