refactor: sol! AST and macro expansion

[DaniPopes]

Decouple expansion logic from the AST.

• ast
  • item (previous input)
    • Struct
    • Error
    • ...
  • everything else
• expand
  • previous ToTokens implementations, but as separate functions

Additionally, refactor attributes parsing into doing the same as syn (parse attributes before parsing an Item, then append them into the item's).
Input -> SingleInput -> InputKind becomes File -> Item.

[prestwich]

let's take this opportunity to document our parser's relationship to the grammar, for future reference

[prestwich]

also to consider:

• do we intend this to grow into a full solidity parser ever?
• should the AST be in a separate crate?

[DaniPopes]

Probably yes, in the future, same for the traits. Would be too much for this PR and we don't need it yet

[prestwich]

Probably yes, in the future, same for the traits. Would be too much for this PR and we don't need it yet

let's open an issue for it :)