* Add general purpose `setFileSize` (unexported for now). Use to simplify
`memfiles.open` as well as make robust (via hard allocation, not merely
`ftruncate` address space allocation) on systems with `posix_fallocate`.
As part of this, fix a bad `closeHandle` return check bug on Windows and
add `MemFile.resize` for Windows now that setFileSize makes that easier.
* Adapt existing test to exercise newly portable `MemFile.resize`.
* Since Apple has never provided `posix_fallocate`, provide a fallback.
This is presently written in terms of `ftruncate`, but it can be
improved to use `F_PREALLOCATE` instead, as mentioned in a comment.
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