Proposal to make `serde_derive_internals::attr::Attr` and `serde_derive_internals::symbol::Symbol` public

[hasezoey]

This is a proposal to make the type Attr public (and by extension Symbol)

Why

I am currently writing a derive add-on and i would like to not re-invent the wheel when the implementations already exist and can be "generically" used

Implementation

To implement the proposal, the following things have to be done:

• [ ] make symbol::Symbol public
• [ ] make attr::Attr public (cant be done without the above)