Support assertion functions with assertion signatures, `assertLength` and others

[tamuratak]

Hi,
it would be useful to support assertion function with assertion signatures, assertLength, assertMinLength, and assertNonEmpty. The implementation would be:

export function assertLength<T, N extends number>(
    arr: readonly T[],
    length: N,
): asserts arr is ReadonlyArrayExactLength<T, N> {
    if (!hasLength(arr, length)) {
        throw new Error(`Length is expected to be equal to ${length}, got ${arr.length}`)
    }
}

• https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions
• Assertions in control flow analysis microsoft/TypeScript#32695