Move rondpoint to fixtures, remove "roundtripping" stuff from coveralls.

[mhammond]

Via https://github.com/mozilla/uniffi-rs/pull/431#issuecomment-835360035 and @jhugman, a possible improvement might be:

• lean into the fact that rondpoint really is more a "fixture" than an example.
• acknowledge that for some strange reason I added some roundtripping stuff to coveralls, which doesn't seem to add coverage that rondpoint doesn't already cover.

So maybe we could:

• Move rondpoint to fixtures and have it be the canonical "simple types" (or some better framing) test fixture.
• Have coveralls be more "advanced" (or some better framing) test fixture, and remove most of the simple type stuff.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-59