Deterministic cfbs

[sftse]

I need a way to reliably generate deterministic cfb files. It seems it suffices to be able to override the timestamps.

[mdsteele]

Thanks, these seem like good methods to add.

[sftse]

This is a bit beside the point of this PR, but I happen to notice that the timestamps coming from Entry::created and Entry::modified are nonsensical on streams since those timestamps are all zeroes. Should these functions perhaps return Option<SystemTime>?

[sftse]

@mdsteele
The comments you raised were addressed, is there anything else to be done?

[mdsteele]

Thanks! (Sorry for the long delay, this had fallen off my radar.)