Remove `ArrayLike` from `ObservableInput`

[benlesh]

Related to this issue here: #4563

The problem is that this is true:

const x: ArrayLike<any> = () => {}

But at runtime, our check makes sure it's not a function:

export const isArrayLike = (<T>(x: any): x is ArrayLike<T> => 
    x && typeof x.length === 'number' && typeof x !== 'function');

This, combined with the fact that any "array-like" I know of has implemented Symbol.iterator now, means that this type is mostly worthless. I doubt many people are like:

const source$ = from({
   length: 2,
   "0": "wee",
   "1": "I'm weird!"
})

[benlesh]

Okay, this gets weirder and weirder... in TS 4.1.3:

interface ArrayLike<T> {
    readonly length: number;
    // readonly 0: T;
    readonly [n: number]: T;
}

function works(arg: ArrayLike<any>) {}

works(() => {});

function doesNotWork<T>(arg: ArrayLike<T>) { }

doesNotWork(() => {});

I'm not totally sure what to make of that... playground here

[cartant]

Related TS issue: microsoft/TypeScript#18757

[benlesh]

We probably shouldn't do this for now... this PR in TypeScript is of interest: microsoft/TypeScript#41660