Lwt support

[g2p]

I'd like to use Benchmark to benchmark within an Lwt main loop.

I can't just benchmark a function that calls Lwt_main.run because I have some expensive initialisation done within the Lwt monad, so I need to add the functionality to Benchmark itself. Feel free to come up with advice if you want.

[Chris00]

Have not thought much about it but how can you measure the time spend by a function if the computation can switch to another function in the process? Some help from Lwt is needed I think.