Fix some golangci-lint warnings (#4448)

lite/helpers.go

@@ -42,7 +42,7 @@ func (pkz privKeys) Extend(n int) privKeys {
 }
 
 // GenSecpPrivKeys produces an array of secp256k1 private keys to generate commits.
-func GenSecpPrivKeys(n int) privKeys {
+func genSecpPrivKeys(n int) privKeys {
 	res := make(privKeys, n)
 	for i := range res {
 		res[i] = secp256k1.GenPrivKey()
@@ -52,7 +52,7 @@ func GenSecpPrivKeys(n int) privKeys {
 
 // ExtendSecp adds n more secp256k1 keys (to remove, just take a slice).
 func (pkz privKeys) ExtendSecp(n int) privKeys {
-	extra := GenSecpPrivKeys(n)
+	extra := genSecpPrivKeys(n)
 	return append(pkz, extra...)
 }