Also refactor and document why this hoisting is needed.
This change makes the user_presence test fail because the hoisting
exposes an error encoded into the test's expected result.
Previously, the test expected 2 members in the room at the end. This is
incorrect since one of the members in the test data leaves the room.
However, since the prev_content of state events was previously not
hoisted to the top level, the `membership_change` method would not
notice it and thus not realize the member had left the room. The test
was corrected to expect only a single member in the room.
Another test change was made due to a limitation of EventBuilder: due to
the fact that it makes the test data go through a de/ser cycle, it
cannot easily hoist prev_content to the top level. Because of this, the
tests were change to put prev_content into the top level from the
outset.
03e53e991b