This is currently speculative, since MK v0.10.5 has not been
released yet. Nonetheless, this functionality should be working
when we have relased that version.

Psiphon is not using `go.mod`. Updating also psiphon should be
done with extra care precisely because of that.

For other dependencies, we can go YOLO.

This diff introduces code to redact IP, ASN, and CC directly
in the engine, rather than delegating doing that.

When you initialise a Session, it's your responsibility to
change the default privacy settings.

The names of the collector.PrivacySettings structure fields
are the same names used by ooni/probe-cli.

After a measurement, the settings configured in the session are
honoured with respect to redacting IP, ASN, and CC.

We take an extra step of redacting the whole test keys
string, _if_ it contains the IP.

This is an extra safety net to make sure that we are really
scrubbing the body. The nettest should do that.

This commit is part of #28.

1. make sure we use a separate structure for the open report response
such that the server cannot write our data structure

2. make sure symbols that can be private are actually private

3. make sure the code is compliant with the spec by checking that
the server actually supports `json` reports