...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'.
Prior to this commit, a 'json!' invocation returned a value of type
'Value' from 'serde_json'. Because 'Value' does not implement
'Responder', most uses of 'json!' were wrapped in 'Json':
'Json(json!(..))`. By returning a crate-local 'JsonValue' type that
implements 'Responder', this repetition is resolved, and a 'json!' can
appear unwrapped.
This commit also removes the reexport of 'Value' from 'rocket_contrib'
as well as the default type of 'Value' for 'T' in 'Json<T>'.
