Add beginning of complex serialization / deserialization tests (#6834)

[Wcubed]

Objective

Lay down the basis for catching serialization / deserialization bugs, such as those listed in #6834.

Solution

Added a backend-agnostic roundtrip test, along with implementations for ron, postcard, bincode and messagepack.

[Wcubed]

I created a separate crate for adding integration tests, so we can separate out the dependencies for the integration tests with those for the examples.

I was not sure whether to put the tests all in the src directory, or all in the tests directory. If we put them in the src directory, we could re-use components we create for tests in different tests. Or have I missed something and is it possible to include one test module in another?

[Wcubed]

Rebased upon current master.