The core 'Rocket' type is parameterized: 'Rocket<P: Phase>', where
'Phase' is a newly introduced, sealed marker trait. The trait is
implemented by three new marker types representing the three launch
phases: 'Build', 'Ignite', and 'Orbit'. Progression through these three
phases, in order, is enforced, as are the invariants guaranteed by each
phase. In particular, an instance of 'Rocket' is guaranteed to be in its
final configuration after the 'Build' phase and represent a running
local or public server in the 'Orbit' phase. The 'Ignite' phase serves
as an intermediate, enabling inspection of a finalized but stationary
instance. Transition between phases validates the invariants required
by the transition.
All APIs have been adjusted appropriately, requiring either an instance
of 'Rocket' in a particular phase ('Rocket<Build>', 'Rocket<Ignite>', or
'Rocket<Orbit>') or operating generically on a 'Rocket<P>'.
Documentation is also updated and substantially improved to mention
required and guaranteed invariants.
Additionally, this commit makes the following relevant changes:
* 'Rocket::ignite()' is now a public interface.
* 'Rocket::{build,custom}' methods can no longer panic.
* 'Launch' fairings are now 'ignite' fairings.
* 'Liftoff' fairings are always run, even in local mode.
* All 'ignite' fairings run concurrently at ignition.
* Launch logging occurs on launch, not any point prior.
* Launch log messages have improved formatting.
* A new launch error kind, 'Config', was added.
* A 'fairing::Result' type alias was introduced.
* 'Shutdown::shutdown()' is now 'Shutdown::notify()'.
Some internal changes were also introduced:
* Fairing 'Info' name for 'Templates' is now 'Templating'.
* Shutdown is implemented using 'tokio::sync::Notify'.
* 'Client::debug()' is used nearly universally in tests.
Resolves#1154.
Resolves#1136.
...because loading up a Rocket while it's ignited is a bad idea.
More seriously, because 'Rocket.ignite()' will become an "execute
everything up to here" method.
The attribute is applied everywhere it can be across the codebase and is
the newly preferred method for launching an application. This commit
also makes '#[rocket::main]` stricter by warning when it is applied to
functions other than 'main'.
