This commit adds support for a BrokenTcp provider that can make
connection attempts fail or time out.  It doesn't yet have a way to
turn on the failure.

Code motion and the minimal mechanical changes.

As per
  !375 (comment 2783078)

Empty crate right now

This commit adds a new program to try to implement the ideas behind
experimentation in #329.  In particular, it tries to implement
basic client "can I bootstrap and connect" functionality testing,
with a lot of instrumentation, and support for breaking things.

So far, the instrumentation is limited to counting TCP bytes and
connections, and counting events.  Still, this is enough to measure
behavior on some of the incorrect-clock tests.

NOTE:

For now, you are _required_ to pass in an explicit configuration, in
hopes that this will lead you to override your storage directories
for doing specific experiments.

We now print slighly more information.

Preparatory work for getting rid of some manual Debug impls

Apropos discussion in
  tpo/core/arti!274

Add tls_conn field to ArtiHttpConnector (and argument to constructor).
Introduce MaybeHttpsStream and use it in ArtiHttpConnection.
Have the example program pass the native TLS connector.

Currently the TLS connector and the HTTPS variant are not used, but
this commit is very noisy and fomrulaic, so I have split out the code
to use them into a separate commit for easier preparation and review.

(It's good to do this a couple of days in advance in case it turns
up any bugs.  That's how we caught the panic bug in simple_asn1
0.6.0)

We need this not to use anyhow because we don't want our libraries to
expose anyhow, and this is about to go into the library.

Code motion and consequential dependency adjustments.

This isn't complete (see TODO), but it's enough to let us report the
right ErrorKind if something fails to parse.

The new `BootstrapBehavior` enum controls whether an unbootstrapped
`TorClient` will bootstrap itself automatically (`Ondemand`) when an
attempt is made to use it, or whether the user must perform
bootstrapping themselves (`Manual`).

The `lazy-init` example shows how you could write a simple
`get_tor_client()` function that used a global `OnceCell` to share
a Tor client across an entire application with this API.

closes #278

Additionally, refactor the IoError out of tor_cell::Error:
nothing in TorCell created this; it was only used by tor_proto.

This required refactoring in tor_proto to use a new error type. Here I
decided to use a new CodecError for now, though we may refactor that
away soon too.

This error type doesn't impement HasKind, since the kind will depend
on context.

However, the existing implementation was pretty messy and inconsistent:
Some errors had positions, some didn't.
Some took messages as str, some as String.
Some had internal errors that were somewhat orthogonal to their actual
types.

This commit refactors tor_netdoc::Error to use a ParseErrorKind, and
adds a set of convenience functions to add positions and
messages to the errors that need them.

Since the only purpose of this function is to make sure that no
bootstrapping task is running, a simple futures::lock::Mutex
should do the job just fine.

Closes #337.

This commit changes how the `TorClient` type works, enabling it to be
constructed synchronously without initiating the bootstrapping process.
Daemon tasks are still started on construction (although some of them
won't do anything if the client isn't bootstrapped).

The old bootstrap() methods are now reimplemented in terms of the new
create_unbootstrapped() and bootstrap_existing() methods.

This required refactoring how the `DirMgr` works to enable the same sort
of thing there.

closes #293

(This error isn't yet wrapped in TorError, but it will be eventually
when we implement socks proxy and PT support.)

This crate's Error type is too low-level to have an ErrorKind, but
it does make sense to use InternalError for the internal errors
here.

This required a few new ErrorKinds.

This needs two kinds.  We have decided to treat a non-shutdown
SpawnError as "unexplained" rather than as an InternalError.

There are many crates whose
  From<futures::task::SpawnError> for Error
erroneously treat it as an internal error.  We will fix them in a moment.

Doing this here makes it easier when I rebase/reorder things