Avoid unnecessary delay in pMapIterable

Eagerly invoke iterator.next() even when already at concurrency limit.

Otherwise, when a slot frees up and we have the chance to invoke the
mapper, we first have to wait for iterator.next() to resolve. During
this time the free slot is unused.

With this change the resolution of iterator.next() will already have
started and there is at least a chance that it's ready by the time we
want to invoke the mapper.

In principle, one could even implement a configurable backlog for
iterator.next() results. That would be a bigger change though.

Fixes sindresorhus#78

index.js

@@ -190,6 +190,7 @@ export function pMapIterable(
 		async * [Symbol.asyncIterator]() {
 			const iterator = iterable[Symbol.asyncIterator] === undefined ? iterable[Symbol.iterator]() : iterable[Symbol.asyncIterator]();
 
+			let nextIterResult = iterator.next();
 			const promises = [];
 			let pendingPromisesCount = 0;
 			let isDone = false;
@@ -202,7 +203,10 @@ export function pMapIterable(
 
 				const promise = (async () => {
 					pendingPromisesCount++;
-					const {done, value} = await iterator.next();
+
+					const iterResult = nextIterResult;
+					nextIterResult = iterator.next();
+					const {done, value} = await iterResult;
 
 					if (done) {
 						pendingPromisesCount--;