rustdoc-json: Rename `implementors` to `implementations`

[aDotInTheVoid]

The ID's in Trait's implementors field are those for impl Trait for Type, not Type

Eg:

pub trait Debug {}

impl Debug for u8 {}

"0:3": {
      "inner": {
        "implementors": ["0:4"],
        "items": []
      },
      "kind": "trait",
      "name": "Debug",
  }

    "0:4": {
      "id": "0:4",
      "inner": {
        "blanket_impl": null,
        "for": {"inner": "u8", "kind": "primitive"},
        "items": [],
        "trait": {
          "inner": {
            "args": {"angle_bracketed": {"args": [], "bindings": []}},
            "id": "0:3",
            "name": "Debug",
            "param_names": []
          },
          "kind": "resolved_path"
        }
      },
      "kind": "impl",
      "links": {},
      "name": null,
      "span": {"begin": [15, 0], "end": [15, 20], "filename": "blanket.rs"},
      "visibility": "crate"
    },

@rustbot modify labels: +T-rustdoc +A-rustdoc-json