next: Replace Slice type with Iterable type

[elycruz]

Complex sum types cause more trouble than it is usually worth, in a low level library - Where ever we have to cast types, internally, means the same has to happen from the outside (in some cases) - for these case we should use simpler, atomic types.

Example case with casting issue:

// types.ts
export type Slice<T = any> = T[] | string;

// reverse.ts
// ...
export const

  reverse = <T>(xs: Slice<T>): Slice<T> => {
    if (!isset(xs) || !xs.length) {
      return xs;
    }
    let i = xs.length - 1;
    if (typeOf(xs) === 'String') {
      let out = of(xs) as string;
      for (; i >= 0; i -= 1) {
        out += xs[i];
      }
      return out;
    }
    const out = of(xs) as T[];
    for (; i >= 0; i -= 1) {
      out.push(xs[i] as T);
    }
    return out;
  }

// Elsewhere (in tests/etc.)
const range0To145 = range(0, 145, 5),

  descNumList = [99].concat(reverse(range0To145)) // type error:
/*  error TS2769: No overload matches this call.
      Overload 1 of 2, '(...items: ConcatArray<number>[]): number[]', gave the following error.
        Argument of type 'Slice<number>' is not assignable to parameter of type 'ConcatArray<number>'.
          Type 'string' is not assignable to type 'ConcatArray<number>'.
      Overload 2 of 2, '(...items: (number | ConcatArray<number>)[]): number[]', gave the following error.
        Argument of type 'Slice<number>' is not assignable to parameter of type 'number | ConcatArray<number>'.
          Type 'string' is not assignable to type 'number | ConcatArray<number>'.
*/

A better approach to the reverse implementation is to only accept Iterable<T>, return T[], and allow the user to decide how to get their string into the method, and alternately supply a reverseStr` method if the method is really required for strings etc..

Acceptance Criteria

• Remove the slice type and uses.
• Update method implementations, tests, and methods that use the [Slice] type, to only use the Iterable<T> type.

[elycruz]

Issue no longer required - We have found a way to continue using the Slice type by having it extend the platform's Iterable<T> type.