Support non-single-pair types

[CAD97]

Things like how Vec<impl FromPest> works today. FromPest is now set up such that implementations pull from the front of a cursor instead of only getting a single Pair; we should let pest-ast use this!

Basically, this is translating a silent rule from pest.

rule = _{ a ~ b ~ c }

becomes

#[derive(FromPest)]
#[pest_ast(inline(Rule))]
pub struct rule {
    pub a: a,
    pub b: b,
    pub c: c,
}

The struct attribute is temporary and bikeshedable. I don't want just the absence of a rule(..) attribute to mean that it's inline, though, as this is likely to lead to mistakenly leaving it out. The rule enum also still has to be provided somehow, as I don't think it can be inferred from children.