Test oldest example configuration file

See merge request tpo/core/arti!684

tor-config: Provide resolve_option_general, for T: !Default etc.

See merge request tpo/core/arti!685

Mostly cribbed from
  tpo/core/arti!684 (comment 2829937)

At one point in this MR I thought I was going to want this for
arti::cfg::ListenConfig (which we don't want to be Default).

In fact ListenConfig is being handled specially, but having written
this function it seemed sensible to keep it.  Since resolve_option
becomes a wrapper for it, the existing tests exercise it.

The defaults are now

Tolerate RUSTSEC-2021-0139 (ansi_term unmaintained)

See merge request tpo/core/arti!683

This makes it easier to for me to test this script.  (I would like to
do this to all of them but it's easier on a case-by-case basis.)

Format python script in maint

See merge request tpo/core/arti!678

Channel padding

See merge request !657

We do this now.  As per
  !657 (comment 2828364)

Or rather, if we *didn't* negotiate 4, which is too old.

As per
  tpo/core/arti!657 (comment 2828354)

We already actually send and negotiate the padding, since !657,
but we ought to negotiate a protocol version where that's not a
violation!

As per
  tpo/core/arti!657 (comment 2828354)

Making ChannelPaddingInstructions::default() accurately reflect the
initial state of the reactor's padding timer simplifies the code
somewhat.

(When padding is wanted, parameters are computed and inserted
explicitly, so the only change is that if we start out dormant, we
defer setting the timer parameters until necessary.)

As per
  tpo/core/arti!657 (comment 2827249)

As per
  tpo/core/arti!657 (comment 2827249)

As suggested in
  !657 (comment 2827246)

Prompted by
  tpo/core/arti!657 (comment 2827242)

These texts were out of date; one of them still mentioned
`all_zeroes`.

Fixes "chanmgr configuration: Avoid sending needless initial update(s)"

This was for testing and is no longer needed.

This allow us to make a working cross-reference.

As per
  tpo/core/arti!657 (comment 2826169)

As per
  tpo/core/arti!657 (comment 2826168)

Run rustfmt; no other changes.

As per
  tpo/core/arti!657 (comment 2826167)

This makes some lines too long; I will run rustfmt in a separate
commit for clarity.

Reported here
  tpo/core/arti!657 (comment 2826166)

As per
  tpo/core/arti!657 (comment 2826164)