Provide public access to RenameAllRules in `serde_derive_internals`

[uint]

Hi! Through serde_derive_internals I can get access to serde attribute parsers, which is super neat.

The particular data I need though is the rename_all rules, and the exposed struct isn't very useful with its fields private and no way to access them.

https://github.com/serde-rs/serde/blob/b23a76841425c70926efb51d46353cccc45c1145/serde_derive/src/internals/attr.rs#L203-L206