Make API friendly to block explorers (#702)

* Add validator index to duties response

* Add `get_state` method to beacon chain

* Improve /beacon/validators endpoint

* Add validators/all and validators/active endpoints

* Start refactor of HTTP docs

* Document /beacon/heads endpoint

* Remove some unused API endpoints

* Improve API docs

* Add methods to get all validator duties

* Improve docs

* Remove dead links

* Make tables left-justified

* Add /consensus/vote_count endpoint

* Add /consensus/individual_votes endpoint

* Update formatting

* Tidy

* Add committees endpoint

* Strictly require 0x prefix for serde in BLS

* Update docs to have 0x prefix

* Fix failing tests

* Add unfinished code

* Improve testing, fix bugs

* Tidy, ensure all beacon endpoints smoke tested

* Fix pubkey cache error

* Address comments with docs

eth2/types/src/beacon_state.rs

@@ -388,6 +388,19 @@ impl<T: EthSpec> BeaconState<T> {
         cache.get_beacon_committees_at_slot(slot)
     }
 
+    /// Get all of the Beacon committees at a given relative epoch.
+    ///
+    /// Utilises the committee cache.
+    ///
+    /// Spec v0.9.1
+    pub fn get_beacon_committees_at_epoch(
+        &self,
+        relative_epoch: RelativeEpoch,
+    ) -> Result<Vec<BeaconCommittee>, Error> {
+        let cache = self.committee_cache(relative_epoch)?;
+        cache.get_all_beacon_committees()
+    }
+
     /// Compute the proposer (not necessarily for the Beacon chain) from a list of indices.
     ///
     /// Spec v0.9.1

eth2/types/src/beacon_state/committee_cache.rs

@@ -141,6 +141,21 @@ impl CommitteeCache {
             .collect()
     }
 
+    /// Returns all committees for `self.initialized_epoch`.
+    pub fn get_all_beacon_committees(&self) -> Result<Vec<BeaconCommittee>, Error> {
+        let initialized_epoch = self
+            .initialized_epoch
+            .ok_or_else(|| Error::CommitteeCacheUninitialized(None))?;
+
+        initialized_epoch.slot_iter(self.slots_per_epoch).try_fold(
+            Vec::with_capacity(self.slots_per_epoch as usize),
+            |mut vec, slot| {
+                vec.append(&mut self.get_beacon_committees_at_slot(slot)?);
+                Ok(vec)
+            },
+        )
+    }
+
     /// Returns the `AttestationDuty` for the given `validator_index`.
     ///
     /// Returns `None` if the `validator_index` does not exist, does not have duties or `Self` is