Support a variety of attributes

[pczarn]

doc comments on rules in grammar

• support

Example

/// A potentially captured element.
pat_elem ::= pat:pattern elem:elem;

Should result with the following somewhere down the road

#[doc = "A potentially captured element."]
struct PatElem {
    pat: Pattern,
    elem: Elem,
}

inline attrs

• support
• variety of levels

Example

grammar! {
    #![inline(level = 2)]
    ...
}