Consolidate test doubles

- Migrate various mocks of core namespaces to shared version in `fakes` pkg
- Err on the side of making test doubles less sophisticated
- Don't pull over mocks of namespaces that are only used in example code

pkg/core/eth_client.go

@@ -0,0 +1,19 @@
+package core
+
+import (
+	"context"
+	"math/big"
+
+	"github.com/ethereum/go-ethereum"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+)
+
+type EthClient interface {
+	BlockByNumber(ctx context.Context, number *big.Int) (*types.Block, error)
+	CallContract(ctx context.Context, msg ethereum.CallMsg, blockNumber *big.Int) ([]byte, error)
+	FilterLogs(ctx context.Context, q ethereum.FilterQuery) ([]types.Log, error)
+	HeaderByNumber(ctx context.Context, number *big.Int) (*types.Header, error)
+	TransactionSender(ctx context.Context, tx *types.Transaction, block common.Hash, index uint) (common.Address, error)
+	TransactionReceipt(ctx context.Context, txHash common.Hash) (*types.Receipt, error)
+}