Use a drop guard to prevent leaving a dangling reference in internals::serialize::id::run_as_context

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After reading about the recent improvements to Miri, I tried using it to run Legion's tests. Most of them succeed, but internals::serialize::test::serialize_bincode and internals::serialize::test::serialize_json both panic for reasons I haven't looked into, and because they run in the same thread, the second one finds a dangling pointer and triggers an error in Miri.

This pull request fixes that problem, leaving both test results as panics rather than undefined behavior. The comment suggested using catch_unwind for this, but I used the more common and probably more efficient pattern of a guard struct with a Drop implementation that cleans up. I also added a small test to make sure Miri always has the opportunity to catch this bug, since it was essentially a coincidence that it was caught this time.

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Out of the other non-succeeding tests:

• internals::systems::command::test::simple_write_test causes an ICE
• internals::systems::schedule::test::flush causes an ICE
• internals::systems::schedule::test::flush_thread_local causes an ICE
• internals::world::test::pack ran out of memory on my machine