Add the convenience fun Time_ns.since

[theteachr]

Adds a convenience function to compute the elapsed time. Instead of

Time_ns.(diff (now ()) start)

One can do

Time_ns.since start

[nmatschke]

Thanks for the PR. We discussed internally and decided we'd prefer not to merge it - sorry!

In particular, we're concerned that this hides the dependency on now, and makes it harder to reason about code. Also, if you call since twice, you will use two different values of now! While having to spell out now is a bit verbose, it makes the dependency on the system clock more explicit, and makes it easier to spot bugs.