Block reward contract (#8419)

* engine: add block reward contract abi and helper client

* aura: add support for block reward contract

* engine: test block reward contract client

* aura: test block reward contract

* engine + aura: add missing docs

* engine: share SystemCall type alias

* aura: add transition for block reward contract

* engine: fix example block reward contract source link and bytecode

ethcore/res/contracts/block_reward.json

@@ -0,0 +1,29 @@
+[
+  {
+    "constant": false,
+    "inputs": [
+      {
+        "name": "benefactors",
+        "type": "address[]"
+      },
+      {
+        "name": "kind",
+        "type": "uint16[]"
+      }
+    ],
+    "name": "reward",
+    "outputs": [
+      {
+        "name": "",
+        "type": "address[]"
+      },
+      {
+        "name": "",
+        "type": "uint256[]"
+      }
+    ],
+    "payable": false,
+    "stateMutability": "nonpayable",
+    "type": "function"
+  }
+]