This removes almost all uses of the global value.  All that remains is its use
as a default in case no configuration is given.

The new direct positive-path Hypothesis-based test wants to pass in huge
numbers and the float-based implementation does not do well for these inputs.
Happily, we can do better.

Only the documentation was wrong, whew.

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.

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.

Integer division lets this number be zero sometimes.

If enough valid passes are supplied for the amount of storage requested then
allow the operation to succeed.  Otherwise, fail with an error.