Problem: deep context stack efficienty is not benchmarked

Closes: evmos#626

Solution:
- add a benchmark to demonstrate an extremely inefficiency in deep
  context stack

x/evm/keeper/benchmark_test.go

@@ -6,12 +6,14 @@ import (
 
 	"github.com/stretchr/testify/require"
 
+	"github.com/cosmos/cosmos-sdk/store/prefix"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	authante "github.com/cosmos/cosmos-sdk/x/auth/ante"
 	"github.com/ethereum/go-ethereum/common"
 
 	ethtypes "github.com/ethereum/go-ethereum/core/types"
 	ethermint "github.com/tharsis/ethermint/types"
+	"github.com/tharsis/ethermint/x/evm/keeper"
 	"github.com/tharsis/ethermint/x/evm/types"
 )
 
@@ -95,3 +97,46 @@ func BenchmarkTokenMint(b *testing.B) {
 		return types.NewTx(suite.app.EvmKeeper.ChainID(), nonce, &contract, big.NewInt(0), 410000, big.NewInt(1), nil, nil, input, nil)
 	})
 }
+
+func DoBenchmarkDeepContextStack(b *testing.B, depth int) {
+	begin := []byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
+	end := []byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}
+
+	suite := KeeperTestSuite{}
+	suite.DoSetupTest(b)
+
+	transientKey := suite.app.GetTKey("transient_evm")
+
+	var stack keeper.ContextStack
+	stack.Reset(suite.ctx)
+
+	for i := 0; i < depth; i++ {
+		stack.Snapshot()
+
+		store := prefix.NewStore(
+			stack.CurrentContext().TransientStore(transientKey),
+			types.KeyPrefixTransientTxLogs,
+		)
+		store.Set(begin, []byte("value"))
+	}
+
+	store := prefix.NewStore(
+		stack.CurrentContext().TransientStore(transientKey),
+		types.KeyPrefixTransientTxLogs,
+	)
+	for i := 0; i < b.N; i++ {
+		store.Iterator(begin, end)
+	}
+}
+
+func BenchmarkDeepContextStack1(b *testing.B) {
+	DoBenchmarkDeepContextStack(b, 1)
+}
+
+func BenchmarkDeepContextStack10(b *testing.B) {
+	DoBenchmarkDeepContextStack(b, 10)
+}
+
+func BenchmarkDeepContextStack13(b *testing.B) {
+	DoBenchmarkDeepContextStack(b, 13)
+}