Function mergeByKey

mergeByKey<V>(
    keyFunction: (value: V) => string,
    reconcile: Data.Arrays.MergeReconcile<V>,
    ...arrays: readonly (readonly V[])[],
): V[]
Merges arrays left to right, using the provided reconcile function to choose a winner when keys overlap.

There's also Data.Maps.mergeByKey if the input data is in Map form.

For example, if we have the array A: [A-1, A-2, A-3]

And array B: [B-1, B-2, B-4]

And with the key function:
```
// Make a key for value based on last char
const keyFn = (v) => v.substr(-1, 1);
```
If they are merged with the reconile function:
```
const reconcile = (a, b) => b.replace(`-`, `!`);
const output = mergeByKey(keyFn, reconcile, arrayA, arrayB);
```
The final result will be:

[B!1, B!2, A-3, B-4]

In this toy example, it's obvious how the reconciler transforms data where the keys overlap. For the keys that do not overlap - 3 and 4 in this example - they are copied unaltered.

A practical use for mergeByKey has been in smoothing keypoints from a TensorFlow pose. In this case, we want to smooth new keypoints with older keypoints. But if a keypoint is not present, for it to be passed through.
Type Parameters
- V
Parameters
- keyFunction: (value: V) => string
  Function to generate a unique key for data
- reconcile: Data.Arrays.MergeReconcile<V>
  Returns value to decide 'winner' when keys conflict.
- ...arrays: readonly (readonly V[])[]
  Arrays of data to merge
Returns V[]