Auto merge of #55906 - nnethercote:rm-OpenSnapshot-CommittedSnapshot, r=nikomatsakis

Clean up and streamline snapshot data structures

These commits clean up the snapshot structures a bit, so they are more consistent with each other and with the `ena` crate.

They also remove the `OpenSnapshot` and `CommittedSnapshot` entries in the undo log, just like I did for the `ena` crate in https://github.com/rust-lang-nursery/ena/pull/14. This PR in combination with that `ena` PR reduces instruction counts by up to 6% on benchmarks.

r? @nikomatsakis. Note that this isn't quite ready for landing, because the `ena` dependency in the first commit needs to be updated once https://github.com/rust-lang-nursery/ena/pull/14 lands. But otherwise it should be good.

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