The documentation and implementation diverged; the documentation was
correct.

With this patch we don't consider a consensus to be even potentially
well-signed if the authorities that are listed as signing it don't
contain enough authorities we believe in.  Otherwise, we'd just try
fetching certs for them and failing forever.

(I found this by switching from a chutney network to the main
network without cleaning out my cache.)

Closes #44

We should find out if we believe in the authorities before we even
think of downloading their certs.

Also replace some "disk" with "cache" in log messages

Based rust nightly cargo fmt.

Now we are testing decompression of the input via calling
the process method 3 times. We get the final decompressed output
only after the third call.

Tests the decompression of the whole input in one go.

Adds dependency to zstd crate.

asynchronous_codec is a fork of futures_codec that is up-to-date on
its dependencies.

This migration allows us to upgrade to the current version of the
bytes crate.

Co-authored-by: David Goulet <dgoulet@torproject.org>

Co-authored-by: David Goulet <dgoulet@torproject.org>

These necessitated a little complexity for our circuit
creation/extension API; we might want to refactor that down the
road.

Closes arti#49.