Introduce DeriveInfo

[pvdrz]

This PR introduces a new non-exhaustive struct called DeriveInfo to be used as the sole argument of ParseCallbacks::add_derives with the purpose of being able to extend the information passed to this method in a backwards-compatible manner, meaning that adding new fields to DeriveInfo won't be a breaking change when releasing a new version.

Some candidates to new fields have been requested already:

• Support for zerocopy #2168: Requests for a way to know if a type is PoD and can be safely transmuted from/to bytes.
• Implement cli option for custom derive #2328: Could be simplified if the kind of the type (whether a type is a struct, enum or union) were included.