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>'.
