Add a "rune inspect" subcommand for inspecting runes

[Michael-F-Bryan]

Continuing from #166, this adds a rune inspect sine.rune sub-command that lets you find out more about a Rune, in particular the Capabilities it uses.

$ rune inspect person_detection.rune
Compiled by: rune v0.2.1 (c846a5b 2021-05-27)
Capabilities:
  image (Image)
    Outputs:
    - u8[1, 96, 96]
    Parameters:
    - height: Int(96)
    - width: Int(96)
    - pixel_format: String("@PixelFormat::GrayScale")

There is also a much richer JSON format you can use:

$ rune inspect person_detection.rune --format json
{
  "build_info": {
    "timestamp": "2021-05-27T22:21:29.802283431Z",
    "profile": "debug",
    "optimization_level": 0,
    "crate_info": {
      "name": "rune",
      "version": "0.2.1",
      "authors": [
        "Kartik Thakore <kartik@thakore.ai>",
        "Akshay Sharma <akshay@sharma.ai>",
        "Michael-F-Bryan <consulting@michaelfbryan.com>"
      ],
      "license": "Apache 2.0",
      "enabled_features": [],
      "available_features": [],
      "dependencies": []
    },
    "compiler": {
      "version": "1.54.0-nightly",
      "commit_id": "881c1ac408d93bb7adaa3a51dabab9266e82eee8",
      "commit_date": "2021-05-08",
      "channel": "Nightly",
      "host_triple": "x86_64-unknown-linux-gnu",
      "target_triple": "x86_64-unknown-linux-gnu"
    },
    "version_control": {
      "Git": {
        "commit_id": "c846a5b8d058296176a0bf8df043a4b8e19f374c",
        "commit_short_id": "c846a5b",
        "commit_timestamp": "2021-05-27T22:05:25Z",
        "dirty": true,
        "branch": "feature/inspect",
        "tags": []
      }
    }
  },
  "rune": {
    "capabilities": {
      "image": {
        "capability_type": "Image",
        "outputs": [
          {
            "type": "u8",
            "dimensions": [
              1,
              96,
              96
            ]
          }
        ],
        "parameters": {
          "width": 96,
          "pixel_format": "@PixelFormat::GrayScale",
          "height": 96
        }
      }
    }
  }
}