impl_standard_builder followup

See merge request tpo/core/arti!505

I have Plans for this macro.  In particular:

 * I have a wip branch which tests that the Builder can be
   deserialised from an empty config (ie, that config reading
   of a config with a blank section for this item works).

 * I think we should autogenerate $Config::builder(),
   and promote that, rather than $ConfigBuilder::default().
   This macro could do that.

This macro is kind of derive-y.  Also it has a test in it, and failing
to call it could allow bugs to exist, as well as missing bits of API.
Putting it next to the structs makes it easy to see that it's actually
been called.

Teach DirMgr to use slightly untimely directories

Closes #467 and #412

See merge request tpo/core/arti!500

arti-config: Move cmdline to tor-config

See merge request tpo/core/arti!498

Improvements prompted by clippy, and disable one lint

See merge request tpo/core/arti!497

Provide and use macro for impl Derive via Builder

See merge request tpo/core/arti!499

Add correct serde(default) attrs for humantime_serde::option

See merge request tpo/core/arti!502

Discovered by a test case in my local tree.  The test case was
macro-generated by an extension of impl_standard_builder (which
macro istself currently awaiting review, arti!499)

Have also sent an MR to update the upstream docs
  https://github.com/jean-airoldie/humantime-serde/pull/8

The type of ret.map_err(codec_err_to_chan)? is ().  ISTM that
writing `let () = ` makes it clear that there is nothing there,
but the lint forbids this.

This lint is warn by default and trips here for me on current nightly.

It seems wrong to me.  We should be able to make it clear to the
reader that there is nothing here - note how this differs from the
lines below where Ready contains msg.  A let () binding is a good way
to do that.

I think the lint allow ought to be added everywhere, but that doesn't
seem easy right now - see this issue about maint/add_warning:
   tpo/core/arti#469

This does involve unwrap, but of course that can't fail unless the
formats fail, which would already panic (that's implied by format!).

This does not know anything about arti, only about TOML and Config.

Code motion, plus necessary import adjustments.

If we're happy with a directory from 3 days ago, we should say
"if-modified-since 3 days ago".

This patch is larger than I'd like, since I had to add &DirMgrConfig
as an argument to the functions that make a  consensus request.

Closes #467.

In our status reporting code, we consider an
expired-but-still-usable directory still bootstrapped, but not 100%
bootstrapped.

Since we want to be willing to use older consensuses, we don't
necessarily want to reset a download just because the consensus is
expired.

This new behavior isn't ideal either; I've added a TODO that relates
to #433.

Related of #412

This new section describes how much variance we accept when it comes
to expired and not-yet-valid directory documents.  (Currently, the
only ones where this matters for are consensus documents and
authority certificates.)  A document that is invalid by no more than
these tolerances is not _live_, but it can still be used.

These tolerances serve two purposes:

 * First, they allow clients to run with a little more clock skew
   than they would tolerate otherwise.
 * Second, they allow clients to survive the situation where the
   authorities are unable to reach a consensus for a day or two.

Compare with Tor's REASONABLY_LIVE_TIME and NETWORKSTATUS_ALLOW_SKEW
constants; also compare with proposal 212.

Closes #412.

Tidy up ConfigurationSource a bit

See merge request tpo/core/arti!495

This will let other embedders use it.

These generic arguments weren't consistent.  It doesn't make sense ot
insist on getting a borrowed type and then cloning it.  So tidy things
up in the direction of taking owned values, which is what
ConfigurationSources actually needs.

(My personal preference would be &dyn to avoid monomorphisation code
bloat but that was controversial last time I proposed it somewhere.)

Discarding this error is not right.

It is not clear to me how this `pub enum` survived the "inaccessible
pub" lint.

This emphasises its nature.  We're going to provide a more cooked
constructor in a moment.

Improve documentation around Cargo features; make Runtime require Debug

Closes #445

See merge request tpo/core/arti!496

- #445 highlighted the lack of good documentation around Arti's
  multiple runtime support, as well as it being difficult to determine
  what runtime was actually in use.
- Improve the documentation to solve the first problem.
- To solve the second problem, make Runtime require Debug (which is
  arguably a good idea anyway, since it makes them easier to embed in
  things), and print out the current runtime's Debug information when
  arti is invoked with `--version`.
- (It also prints out other Cargo features, too!)

fixes #445

Refactor the tor-dirmgr bootstrapping code more gracefully

See merge request tpo/core/arti!488

Disable fs-mistrust in coverage CI.

See merge request tpo/core/arti!493

Remove cargo-husky, and replace with manual instructions

See merge request tpo/core/arti!494

A build script reaching into your .git/hooks/ and modifying them
nonconsensually was a bit of a horrifying concept, and also made it hard
to build arti with the feature disabled. Remove this crate, and replace
it with manual instructions on how to install the hooks in
CONTRIBUTING.md.

- Some FIXMEs got removed or amended.
- AddMicrodescs now yields a mutable reference, so we can use .drain()
  and reuse the allocation.
- Some panics were downgraded to debug_asserts.

- We don't want to inadvertently replace our netdir with one that's
  actually older, so detect and error on this condition.
- Also, print a debug line when we get a new netdir without enough
  guards.
- (An unrelated TODO was also added.)