Add a manifest representation.

[coollog]

Usable representation as well as JSON serialization representation (compatible with #30).

Example manifest.json:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "config": {
    "mediaType": "application/vnd.docker.container.image.v1+json",
    "size": 631,
    "digest": "sha256:41f7343603a17fe746c4bbd5af990b41140edea48be836d0815d57934ca416f5"
  },
  "layers": [
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 1991435,
      "digest": "sha256:b56ae66c29370df48e7377c8f9baa744a3958058a766793f821dadcb144a4647"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 32,
      "digest": "sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 3188459,
      "digest": "sha256:fb0a3ca3d558d9b84913370b909cd3bf8a0debeed7da9e89edd866540b7af214"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 60824735,
      "digest": "sha256:1364c279eb579d95fdcf397e5aeed8a76e71cfab3fb380da4f1c76d786d544fd"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 16786117,
      "digest": "sha256:400deb1d7715413689edfa8f87b1052f74db533c8d47e81b5135562e0dccdd1a"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 155,
      "digest": "sha256:ee73c4ceff1eb920cf5e3df959a159fc70a56b95b044dfb5c658ed09ec2bdabb"
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 4453,
      "digest": "sha256:f28ea763ad4d4191a5d6923d89e034dc8ce4100bd6f401b85d309007d38342e8"
    }
  ]
}