Bugfix / Inconsistency: Respect Uint8Array offsets

[trgwii]

Okay, so found a potential bug in std:

import { varnum } from "https://deno.land/std@0.91.0/encoding/binary.ts";

const main = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8]);
const sub = main.subarray(4, 8);
console.log(varnum(sub, { dataType: "uint8" })); // logs 1

This PR should only affect varnum() and friends operating on Uint8Array subarrays where the offset is > 0.

I replaced all hardcoded 0 indices in the relevant functions with b.byteOffset to get the desired behavior of logging 5 in the above example.

I also added a test for my change (note that this only tests varnum, and not all the relevant functions).

[CLAassistant]

All committers have signed the CLA.

[lucacasonato]

LGTM, thanks!