gh-129005: Align FileIO.readall() allocation (#129458)

Both now use a pre-allocated buffer of length `bufsize`, fill it using
a readinto(), and have matching "expand buffer" logic.

On my machine this takes:

`./python -m test -M8g -uall test_largefile -m test_large_read -v`

from ~3.7 seconds to ~3.4 seconds.