The sane_defaults() call is now the same as you get from a default
builder: by convention, we just call that method Default::default().
The with_directories() constructor makes more sense as a constructor
for the TorClientConfigBuilder than for TorClientConfig.
This overhauls the top-level `arti-client` documentation significantly:
- the "Using arti-client" section walks the user through all of the
necessary steps to initiate a Torified TCP connection, and then
provides a code example
- this example is also available as `examples/readme.rs`; it's not run
as a doctest, since it involves connecting to Tor
- a "More advanced usage" subheading provides information about stream
isolation (and can potentially be used for other interesting
features once we get them).
- a new "Multiple runtime support" section was added to explain the
purpose and usage of the `tor-rtcompat` crate
- the section on design and privacy considerations was removed; this is
probably okay to keep in a README, but users of the crate aren't going
to be interested in this (at least I don't think)
(also, the doc comment for `arti_client::Error` was fixed to make actual
sense)