Utilize the new asserts feature of TS 3.7

[lazarljubenovic]

From here:

function assertIsString(val: any): asserts val is string {
    if (typeof val !== "string") {
        throw new AssertionError("Not a string!");
    }
}

Brainstorm todo list

• Check if it's convenient to use the library with a simple consumer-created assert method, such as assert(tg.isString), or should type-guards provide the assert function with an easy-to-use signature with type guards provided by the library.
• Is it needed to provide things like tg.assertIsString, or should the above assert(tg.isString) or tg.assert(tg.isString) enough?