alias amino imports (#3219)

As per conversation here: https://github.com/tendermint/tendermint/pull/3218#discussion_r251364041

This is the result of running the following code on the repo:

```bash
find . -name '*.go' | grep -v 'vendor/' | xargs -n 1 goimports -w
```

crypto/merkle/proof_test.go

@@ -4,7 +4,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"github.com/tendermint/go-amino"
+	amino "github.com/tendermint/go-amino"
 	cmn "github.com/tendermint/tendermint/libs/common"
 )
 

crypto/merkle/wire.go

@@ -1,7 +1,7 @@
 package merkle
 
 import (
-	"github.com/tendermint/go-amino"
+	amino "github.com/tendermint/go-amino"
 )
 
 var cdc *amino.Codec