Ensure we're handling these attributes correctly when deriving:

• skip
• skip_serializing
• skip_serializing_if

cargo outputs the following warning due to an old Syn version used:

warning: the following packages contain code that will be rejected by a future version of Rust: syn v0.13.11

serde_derive_internals uses the same old Syn version.

Are there any plans regarding updating the dependencies of rust-serde-schema and rust-gob? Should I make a pull-request?

This should also include custom bounds (https://serde.rs/container-attrs.html#serdebound--t-mytrait).

Currently this will stack overflow. Either forbid recursive datastructures completely, or allow serializers/schemas to specify whether recursion is supported, and gracefully handle it.