Do not allow redefinition of existing table

[moorereason]

Given this invalid example from the TOML spec:

[fruit]
apple.color = "red"
[fruit.apple]  # INVALID

toml11 accepts the document as valid:

$ echo "[fruit]\napple.color = \"red\"\n[fruit.apple]" | check_toml_test | jq
{
  "fruit": {
    "apple": {
      "color": {
        "type": "string",
        "value": "red"
      }
    }
  }
}

Tested with commit bf2384d.

PS - I've submitted new tests cases to the toml-test suite for this.

[ToruNiina]

Fixed in e60442c. Thank you!