Also fix a bug in calculating have_enough_paths(): we were assuming
the wrong behavior for weight_for_role().

Rust 1.52 just came out, and there are new clippy lints to deal
with:

   * It spots more cases when we could use Option::map
   * It spots more cases when we could use Iterator::flatten
   * When we build a struct instance, it wants us to list the fields
     in the same order that the struct declares them.

Clippy started warning on this with Rust 1.52.  I'm fairly sure that
when I wrote this code I was worried that <u16 as FromStr>::parse()
might allow hex or something.  But fortunately, it doesn't work that
way, so I don't need to be paranoid here.

Important refactoring happened recently which broke the
"experimental-api" feature.

Fixes are quite simple.
Signed-off-by: David Goulet <dgoulet@torproject.org>

Since these parts are testing-only, let's take steps to make sure we
don't ship them in production by accident.

Now that we have MockNetRuntime and MockSleepRuntime, we can use
them to test a channel-builder by replaying the contents of a TLS
stream that it will accept.

It turns out that we want to take `now` from the Runtime, to make
higher level code testable.