Function map

map<V, R>(mapper: (item: V) => Awaitable<R | undefined>): Task<V, R>
Creates a task converting each item into a new value.

Items are converted lazily, one at a time, and emitted in source order.
Type Parameters
- V
  The type of input items
- R
  The type of output items
Parameters
- mapper: (item: V) => Awaitable<R | undefined>
  The function converting each item; an undefined result contributes nothing, so mapping doubles as filtering
Returns Task<V, R>
A task yielding the values mapper converts the items into
Example
```
await pipe(
  (items(1, 2, 3))
  (map(n => n*2))
  (toArray())
);  // [2, 4, 6]
```
- Defined in tasks/map.ts:44