Fix some cases of unsatisfiable schemas when flattening enums

[GREsau]

Addresses #165

With this change, the output from the example in #165 is:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "MyStruct",
  "type": "object",
  "allOf": [
    {
      "oneOf": [
        {
          "type": "object",
          "properties": {
            "enum1variant": {
              "type": "string"
            }
          },
          "required": [
            "enum1variant"
          ]
        }
      ]
    },
    {
      "oneOf": [
        {
          "type": "object",
          "properties": {
            "enum2variant": {
              "type": "string"
            }
          },
          "required": [
            "enum2variant"
          ]
        }
      ]
    }
  ]
}

Note that it correctly contains both enums, and neither of them have additionalProperties

[GREsau]

This PR now also addresses #164 - the output from that issue's example is:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "MyStruct",
  "type": "object",
  "properties": {
    "property": {
      "type": "string"
    }
  },
  "oneOf": [
    {
      "type": "object",
      "properties": {
        "variant": {
          "type": "string"
        }
      },
      "required": [
        "variant"
      ]
    }
  ],
  "required": [
    "property"
  ],
  "unevaluatedProperties": false
}

Or when using draft-07 settings:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "MyStruct",
  "type": "object",
  "properties": {
    "property": {
      "type": "string"
    },
    "variant": true
  },
  "required": [
    "property"
  ],
  "oneOf": [
    {
      "type": "object",
      "properties": {
        "variant": {
          "type": "string"
        }
      },
      "required": [
        "variant"
      ]
    }
  ],
  "additionalProperties": false
}

[GREsau]

Still to do: improve test coverage - could borrow the types from the linked issues. Also include tests for draft-07/openapi3