Add a simple rate limiter

[watzon]

Rate limiting is very necessary on both sides of the HTTP spectrum, so that you can make sure you don't send too many requests, and so that you can keep your API traffic steady. A simple rate limiter, like the one Go provides, would make this task a lot simpler.

I'm mainly putting this here to a) guage community interest and b) see what maintainers think about having this in the vlib before I start working on it.