Validator API (#504)

* Implemented more REST API endpoints.
 - Added many of the endpoints, which return 501 - Not Implemented
 - Created helper function to return a not implemented error
 - Created a parse_pubkey function to get a PublicKey from a hex string
 - Created a HTTP handler for the validator endpoints
 - Started implementing validator/duties endpoint.

* Fleshed out get validator duties.
 - Re-implemented the get validator duties function for the REST API
 - Added an 'as_hex_string' function to FakePublicKey, beacuse it was missing.

* Fixed small caching/state bug.

* Extended to array of API inputs.
 - Created  function for getting arrays from GET parameters.
 - Extended get validator duties function to support array of validator duties.

* Tidy API to be more consistent with recent decisions

* Addressing Paul's comments.
 - Cleaning up function to get list of proposers.
 - Removing unnecessary serde annotations
 - Clarifying error messages
 - Only accept pubkeys if they are '0x' prefixed.

* Fixed formatting with rustfmt.

eth2/utils/bls/src/fake_public_key.rs

@@ -61,6 +61,13 @@ impl FakePublicKey {
         hex_encode(end_bytes)
     }
 
+    /// Returns the point as a hex string of the SSZ encoding.
+    ///
+    /// Note: the string is prefixed with `0x`.
+    pub fn as_hex_string(&self) -> String {
+        hex_encode(self.as_ssz_bytes())
+    }
+
     // Returns itself
     pub fn as_raw(&self) -> &Self {
         self

eth2/utils/bls/src/public_key.rs

@@ -28,7 +28,7 @@ impl PublicKey {
     /// Returns the underlying point as compressed bytes.
     ///
     /// Identical to `self.as_uncompressed_bytes()`.
-    fn as_bytes(&self) -> Vec<u8> {
+    pub fn as_bytes(&self) -> Vec<u8> {
         self.as_raw().as_bytes()
     }