ethclient: add initial implementation of the typed RPC client

ethclient implements the proposed Ethereum Go API. There are no tests at
the moment, a suite that excercises all implementations of the API will
be added later.

ethclient/ethclient_test.go

+package ethclient
+
+import "github.com/ethereum/go-ethereum"
+
+// Verify that Client implements the ethereum interfaces.
+var (
+	_ = ethereum.ChainReader(&Client{})
+	_ = ethereum.ChainStateReader(&Client{})
+	_ = ethereum.ChainHeadEventer(&Client{})
+	_ = ethereum.ContractCaller(&Client{})
+	_ = ethereum.GasEstimator(&Client{})
+	_ = ethereum.GasPricer(&Client{})
+	_ = ethereum.LogFilterer(&Client{})
+	_ = ethereum.PendingStateReader(&Client{})
+	// _ = ethereum.PendingStateEventer(&Client{})
+	_ = ethereum.PendingContractCaller(&Client{})
+)