Add possibility to request compiler's JSON output

[panicbit]

With the JSON output it would be possible to reorder / tweak messages if needed.
E.g. my IRC bot could use it to prefer showing error messages instead of warnings.

[shepmaster]

Do you know how stable the output of the JSON format is? It might be better for the playground itself to use it, instead of scraping the stdout/stderr. Then the next thing would be to figure out how to expose it.

As a heads-up, the /meta/version/ route has no stability guarantees as it's not intended for public use. the /execute route is (unfortunately) stable because so much relies on it. So stable, in fact, that I'd be pretty hesitant to even add fields to it!

[panicbit]

I'm not sure how stable the output format is, but the compiler let's you use it on the stable channel, so I hope it is? 🤞
At any rate, here's what cargo/rustc outputs for the code given in the linked issue (quite verbose IMO):

{
  "message": {
    "children": [
      {
        "children": [],
        "code": null,
        "level": "note",
        "message": "#[warn(dead_code)] on by default",
        "rendered": null,
        "spans": []
      }
    ],
    "code": {
      "code": "dead_code",
      "explanation": null
    },
    "level": "warning",
    "message": "static item is never used: `X`",
    "rendered": null,
    "spans": [
      {
        "byte_end": 40,
        "byte_start": 16,
        "column_end": 29,
        "column_start": 5,
        "expansion": null,
        "file_name": "src/main.rs",
        "is_primary": true,
        "label": null,
        "line_end": 2,
        "line_start": 2,
        "suggested_replacement": null,
        "text": [
          {
            "highlight_end": 29,
            "highlight_start": 5,
            "text": "    static X: u8 = [1,2][4];"
          }
        ]
      }
    ]
  },
  "package_id": "foo 0.1.0 (path+file:///tmp/foo)",
  "reason": "compiler-message",
  "target": {
    "crate_types": [
      "bin"
    ],
    "kind": [
      "bin"
    ],
    "name": "foo",
    "src_path": "/tmp/foo/src/main.rs"
  }
}
{
  "message": {
    "children": [],
    "code": {
      "code": "E0080",
      "explanation": "\nThis error indicates that the compiler was unable to sensibly evaluate an\nconstant expression that had to be evaluated. Attempting to divide by 0\nor causing integer overflow are two ways to induce this error. For example:\n\n```compile_fail,E0080\nenum Enum {\n    X = (1 << 500),\n    Y = (1 / 0)\n}\n```\n\nEnsure that the expressions given can be evaluated as the desired integer type.\nSee the FFI section of the Reference for more information about using a custom\ninteger type:\n\nhttps://doc.rust-lang.org/reference.html#ffi-attributes\n"
    },
    "level": "error",
    "message": "constant evaluation error",
    "rendered": null,
    "spans": [
      {
        "byte_end": 39,
        "byte_start": 31,
        "column_end": 28,
        "column_start": 20,
        "expansion": null,
        "file_name": "src/main.rs",
        "is_primary": true,
        "label": "index out of bounds: the len is 2 but the index is 4",
        "line_end": 2,
        "line_start": 2,
        "suggested_replacement": null,
        "text": [
          {
            "highlight_end": 28,
            "highlight_start": 20,
            "text": "    static X: u8 = [1,2][4];"
          }
        ]
      }
    ]
  },
  "package_id": "foo 0.1.0 (path+file:///tmp/foo)",
  "reason": "compiler-message",
  "target": {
    "crate_types": [
      "bin"
    ],
    "kind": [
      "bin"
    ],
    "name": "foo",
    "src_path": "/tmp/foo/src/main.rs"
  }
}

[panicbit]

This seems to be the definition of format https://github.com/rust-lang/rust/blob/932c736479f43dc8893a924946e4335d8e308c2e/src/libsyntax/json.rs#L94