Add Sema.run utility for easy task processing

[olee]

I think adding the following utility function might simplify working with semaphores a lot

export class Sema {
    public runTask<T>(task: (token?: any) => Promise<T>): Promise<T> {
        return this.acquire().then(token =>
            Promise.resolve(token)
                .then(task)
                .finally(() => sem.release(token))
        );
    }
}

It would allow to simply and safely queue tasks for processing while making sure that acquired tokens are never lost:

const sem = new Sema(3);

// Run many task in parallel limited by sem
for (let i = 0; i < 100; i++) {
    sem.runTask(async () => {
        // do some stuff
    });
}

// Alternatively use Promise.all and map
await Promise.all(items.map(item => sem.runTask(() => processItem(item))))