Embedding sub-rules into the generated Rule enum

[Louis-Proffit]

Similary to what is done in Menhir in OCaml and ANTLR for some targets, could you imagine embedding sub-rules inside the generated enum ? This would avoid the unnecessary interaction with the Pairs API.

For instance, a rule like pair = { ident~ ":" ~ value } would result in a generated enum of Rule::pair { ident:XXX, value:YYY} where XXX and YYY would be structs or enums representing the ident and value respectively.
This would probably need all rules to be mapped to a struct, and all rules with alternatives to be mapped to enums embedding other structs.

The benefit would be a full compile-time safety for our grammar, that the calls to into_inner(), next() and unwrap() of the current API does not provide.