Changes to rest_api (#480)

* Add half-finished rest api changes

* Add basic, messy changes to rest api

* Fix expect() in ApiRequest

* Remove ApiRequest, add route for beacon state

* Tidy rest api, add get state from root

* Add api method for getting state roots by slot

* Add test for URL helper

* Simplify state_at_slot fn

* Add tests for rest api helper fns

* Add extra tests for parse root

* Fix clippy lints

* Fix compile error in rest api

* Update test to new ethereum-types

beacon_node/src/main.rs

@@ -210,7 +210,6 @@ fn main() {
             Arg::with_name("debug-level")
                 .long("debug-level")
                 .value_name("LEVEL")
-                .short("s")
                 .help("The title of the spec constants for chain config.")
                 .takes_value(true)
                 .possible_values(&["info", "debug", "trace", "warn", "error", "crit"])