gh-120754: Refactor I/O modules to stash whole stat result rather than individual members (#123412)

Multiple places in the I/O stack optimize common cases by using the
information from stat. Currently individual members are extracted from
the stat and stored into the fileio struct. Refactor the code to store
the whole stat struct instead.

Parallels the changes to _io. The `stat` Python object doesn't allow
changing members, so rather than modifying estimated_size, just clear
the value.