Now the network fallbacks configuration wants to Deserialize
a Vec<FallbackDirBuilder>, rather than validated Vec<FallbackDir>.

Methods on FallbackListBuilder are as per
  !462 (comment 2797697)
mutatis mutandi for the fact that this struct has only fallbacks in it.

This is where the FallbackList type is.  We are going to want to
provide a builder too, which ought to impl Default.

This means that the default value for the type must be next to the
type.  In any case, it was anomalous that it wasn't.

This commit is pure code motion.

We are going to be using sub-field builders.

This commitid is the current head of my MR branch
  https://github.com/colin-kiegel/rust-derive-builder/pull/253
  https://github.com/ijackson/rust-derive-builder/tree/field-builder
Using the commitid prevents surprises if that branch is updated.

We will require this newer version of derive_builder.  The version
will need to be bumped again later, assuming the upstream MR is merged
and upstream do a release containing the needed changes.

We will need the new version of not only `derive_builder_core` (the
main macro implementation) but also`derive_builder` for a new error
type.

Fix typos

See merge request tpo/core/arti!461

Keep http in license text.

Typos found with codespell.

Remove obsolete files from our state directory.

Closes #282

See merge request tpo/core/arti!457

Rename "deletable" to "obsolete".

Simplify function structure.

Report errors from `metadata()` and `modified()`.

Don't claim that we're going to delete something unless we are.

Comment about making CUTOFF configurable.

Report skew estimates from arti-client

See merge request tpo/core/arti!455

This patch removes files created by older versions of arti, if they
are at least 4 weeks old.

Closes #282

Fix typos

See merge request tpo/core/arti!453

circmgr: back off on preemptive circuits if they fail consistently

Closes #437

See merge request tpo/core/arti!456

Rather than running preemptive circuit construction every 10
seconds, we change it to back off when it is "failing".  (We define
"failing" as creating no new circuits, and as giving at least one
error.)

This change means that we'll have one less reason to hammer the
network when our connectivity is failed for some reason.

Closes #437.
Part of #329.

Now that we have TaskSchedule, we don't need to expose these any
longer.

This feature is similar to ChanProvenance from ChanMgr, except that
we don't yet need to report it outside the crate.  I'm going to use
it to distinguish newly created circuits from existing circuits in
the preemptive circuit builder.

(Also, blame clock skew when it is an explanation of why we cannot
finish a connection.)

Instead of just having a function that recalculates the latest clock
skew, instead recalculate the clock skew when it may have changed,
and notify other processes via a postage::watch.

guardmgr: fix a unit test panic.

See merge request tpo/core/arti!454

Apparently on OSX you are not allowed to construct an Instant that is a
long time before the time when the test is running.

Also, fix the length of a year in this test.

Collect and analyze clock skew information

See merge request tpo/core/arti!450

This simplifies the code a lot.

This time, our estimator discards outliers, takes the mean of what's
left, and uses the standard deviation to try to figure out how
seriously to take our report of skew/not-skew.

These estimates are still not actually used.

fix maint/cargo_audit not propagating error and add temporary exceptions

See merge request tpo/core/arti!452

use rustc 1.60 for coverage

See merge request tpo/core/arti!451

using stable doesn't work for CI because a specific version like 1.60 is installed

(This is just a placeholder; I'm going to make the functions
smarter in the next commit.)