This commit adds script/c++ documentation and fixes a few loose ends.
It also adds tests for corner cases and massively improves error
messages.
This also actually introduces type-compatibility checking and introduces
a new attribute that lets a user override this if they really know what
they are doing. I am not quite sure if we should really let that stay in
- but it can be very convenient to have this functionality.
One test is continuing to fail - the expiry test is very flaky. This is,
I think, caused by delays of the broker store forwarding. I am unsure if
we can actually do anything about that.
