Closes #163

The affected API (getgrouplist) isn't one that we use, directly or
transitively.  We can't just upgrade immediately, since the ctrlc
crate is currently stuck on the older version of nix.

This is an attempt to uplift the trick that tor-circgmr is currently
using, since we'll want it in tor-guardmgr too.

(Also, fix a test)

I'm not thrilled with this code, but I think it's needed to
implement guards correctly.

We'll need `id_pair_is_listed()` to track whether a sampled guard is
(or is not) listed in the consensus.

We'll need `missing_descriptor_for` to see whether we've downloaded
enough microdescs to use a consensus.

Previously I'd hoped to have it only apply to reproducible builds,
but the work of keeping it up-to-date and checking it for drift
seems to have been tripping us up.  Let's try doing it the other way
for now, and we'll see how that goes.

By default rand does not build for WASM browser targets.