Add type checks to TOMLValueConvertible conversions

[stackotter]

Fixes #12

The tests all pass on my machine, and it fixes the fatal crash for me.

The following code snippet now runs and throws a (non-fatal) error as expected:

import Foundation
import TOMLKit // stackotter/TOMLKit ~> 00de25703c71d965e62dec711660c96482c9e1db

let string = """
[[apps.test]]
"""

struct Child: Codable {
    var value: String
}

struct Parent: Codable {
    var apps: [String: Child]
}

try? TOMLDecoder().decode(Parent.self, from: string)

[stackotter]

I accidentally had my indentation set to spaces instead of tabs in my original commit, so I've fixed that now

[LebJe]

@stackotter Thanks for the fix! I will make a new release soon.

[stackotter]

Awesome, thanks for merging so quickly!