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/beacon_chain/src/beacon_chain.rs

@@ -296,7 +296,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     /// It is important to note that the `beacon_state` returned may not match the present slot. It
     /// is the state as it was when the head block was received, which could be some slots prior to
     /// now.
-    pub fn head(&self) -> RwLockReadGuard<CheckPoint<T::EthSpec>> {
+    pub fn head<'a>(&'a self) -> RwLockReadGuard<'a, CheckPoint<T::EthSpec>> {
         self.canonical_head.read()
     }