fix: Allow for Contexts data type to be infered from the key value

[kamilogorek]

Currently, tests are failing due to visitor going more than one level deep and nested objects effectively getting top-level key assigned to them.

Or rather, after more investigation, it simply puts a type field into the flattened other map, as type is not removed; it normally is, as it's used as tag = "type" during deserialization.

tl;dr - we want both variants to be deserializable (currently first example is failing due to missing type key):

{
  "contexts": {
    "device": {
      "name": "fancy name for a fancy server",
      "arch": "amd64",
      "num_cpu": 8
    },
}

{
  "contexts": {
    "device": {
      "type": "device",
      "name": "fancy name for a fancy server",
      "arch": "amd64",
      "num_cpu": 8
    },
}

Closes #483

[loewenheim]

Oof, that's pretty gnarly. Respect for getting this done.