Check that derived traits are needed on structs

[jlapeyre]

Some of the derived traits on structs were added very early in the development. They are infectious, and so are copied to from struct to struct.

We should find which derived traits are unused and remove them.

This sometimes takes a bit of work because if a struct or enum has a derived trait, it's dependencies often need to implement the same trait, and so on.

My inclination is to exempt Debug because it is heavily used when developing. But is unused in released code.

oq3_semantics is probably a good place to start.

Note: I am not certain that there remain any derived traits that can be removed!