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 arti#278