This PR:
* Added the clippy.toml to configure allowable expect / unwrap usage in
tests
* Removed as many expect/allow lines as possible from tests
* moved a bunch of allows to expects where possible
Note: in integration tests, non `#[test]` helper functions are not
covered by this so we had to leave a few lingering `expect(expect_used`
checks around
refactors HistoryCell to be a trait instead of an enum. Also collapse
the many "degenerate" HistoryCell enums which were just a store of lines
into a single PlainHistoryCell type.
The goal here is to allow more ways of rendering history cells (e.g.
expanded/collapsed/"live"), and I expect we will return to more varied
types of HistoryCell as we develop this area.
