How do I download the Quandl codes of all the time-series in a given data feed?

You can download all the time-series codes and their corresponding metadata with a single call by appending /metadata to your API request. 

For example:

https://www.quandl.com/api/v3/databases/EOD/metadata?api_key=YOURAPIKEY

This call will return a zipped CSV file containing codes, names, descriptions, date ranges and the last update time for all time-series in the EOD product.