rustdoc-json: Improve Item::attrs to not use debug representation.

[aDotInTheVoid]

Currently, Item::attrs uses a debug-printing

Stringified versions of parsed attributes on this item.
Essentially debug printed (e.g. #[inline] becomes something similar to #[attr="Inline(Hint)"]).
Equivalent to the hir pretty-printing of attributes.

https://github.com/rust-lang/rust/blame/85abb276361c424d64743c0965242dd0e7b866d1/src/rustdoc-json-types/lib.rs#L123-L126

This isn't particularly friendly to users, who need to deal with an internal (and undocumented 😱) attr representation.

This was changed in #135726, as part of the a larger attribute rework (#131229)

MCVE:

#[repr(C)]
pub struct Foo(i32);

which produces (abridged):

    "1": {
      "attrs": ["#[attr=\"Repr([ReprC])\")]\n"],
      "inner": {
        "struct": {
          "generics": {"params": [], "where_predicates": []},
          "impls": [2, 4, 6, 8, 10, 12, 15, 19, 23, 26, 31, 36, 39],
          "kind": {"tuple": [null]}
        }
      },
      "name": "Foo",
    }

CC @jdonszelmann @obi1kenobi