Function pairwiseReduce

pairwiseReduce<V, X>(
    array: readonly V[],
    reducer: (accumulator: X, a: V, b: V) => X,
    initial: X,
): X
Reduces in a pairwise fashion.

Eg, if we have input array of [1, 2, 3, 4, 5], the reducer fn will run with 1,2 as parameters, then 2,3, then 3,4 etc.
```
const values = [1, 2, 3, 4, 5]
reducePairwise(values, (acc, a, b) => {
 return acc + (b - a);
}, 0);
```
If input array has less than two elements, the initial value is returned.
```
const reducer = (acc:string, a:string, b:string) => acc + `[${a}-${b}]`;
const result = reducePairwise(`a b c d e f g`.split(` `), reducer, `!`);
Yields: `![a-b][b-c][c-d][d-e][e-f][f-g]`
```
Type Parameters
- V
- X
Parameters
- array: readonly V[]
- reducer: (accumulator: X, a: V, b: V) => X
- initial: X
Returns X