Add lint checks: gofmt, goimports, golint

- gofmt - Gofmt checks whether code was gofmt-ed. By default this tool runs
  with -s option to check for code simplification
- goimports - Goimports does everything that gofmt does. Additionally it checks
  unused imports
- golint - Golint differs from gofmt. Gofmt reformats Go source code, whereas
  golint prints out style mistakes
    - checks the uncommented exported functions & types

Update the code to fix the lint checks.

utils/utils.go

@@ -1,3 +1,7 @@
+// Package utils provides methods to work with Hermez custom half float
+// precision, 16 bits, codification internally called Float16 has been adopted
+// to encode large integers. This is done in order to save bits when L2
+// transactions are published.
 //nolint:gomnd
 package utils
 
@@ -23,8 +27,8 @@ func (f16 Float16) Bytes() []byte {
 }
 
 // BigInt converts the Float16 to a *big.Int integer
-func (fl16 *Float16) BigInt() *big.Int {
-	fl := int64(*fl16)
+func (f16 *Float16) BigInt() *big.Int {
+	fl := int64(*f16)
 
 	m := big.NewInt(fl & 0x3FF)
 	e := big.NewInt(fl >> 11)

utils/utils_test.go

@@ -101,17 +101,17 @@ func BenchmarkFloat16(b *testing.B) {
 		BigInt  *big.Int
 	}
 	testVector := []pair{
-		pair{0x307B, newBigInt("123000000")},
-		pair{0x1DC6, newBigInt("454500")},
-		pair{0xFFFF, newBigInt("10235000000000000000000000000000000")},
-		pair{0x0000, newBigInt("0")},
-		pair{0x0400, newBigInt("0")},
-		pair{0x0001, newBigInt("1")},
-		pair{0x0401, newBigInt("1")},
-		pair{0x0800, newBigInt("0")},
-		pair{0x0c00, newBigInt("5")},
-		pair{0x0801, newBigInt("10")},
-		pair{0x0c01, newBigInt("15")},
+		{0x307B, newBigInt("123000000")},
+		{0x1DC6, newBigInt("454500")},
+		{0xFFFF, newBigInt("10235000000000000000000000000000000")},
+		{0x0000, newBigInt("0")},
+		{0x0400, newBigInt("0")},
+		{0x0001, newBigInt("1")},
+		{0x0401, newBigInt("1")},
+		{0x0800, newBigInt("0")},
+		{0x0c00, newBigInt("5")},
+		{0x0801, newBigInt("10")},
+		{0x0c01, newBigInt("15")},
 	}
 	b.Run("floorFix2Float()", func(b *testing.B) {
 		for i := 0; i < b.N; i++ {
@@ -120,7 +120,7 @@ func BenchmarkFloat16(b *testing.B) {
 	})
 	b.Run("NewFloat16()", func(b *testing.B) {
 		for i := 0; i < b.N; i++ {
-			NewFloat16(testVector[i%len(testVector)].BigInt) //nolint:errcheck
+			_, _ = NewFloat16(testVector[i%len(testVector)].BigInt)
 		}
 	})
 	b.Run("Float16.BigInt()", func(b *testing.B) {