Add NewFromInt and NewFromInt32 (#72)

Add NewFromInt and NewFromInt32

README.md

@@ -34,7 +34,7 @@ func main() {
 		panic(err)
 	}
 
-	quantity := decimal.NewFromFloat(3)
+	quantity := decimal.NewFromInt(3)
 
 	fee, _ := decimal.NewFromString(".035")
 	taxRate, _ := decimal.NewFromString(".08875")

decimal.go

@@ -87,6 +87,32 @@ func New(value int64, exp int32) Decimal {
 	}
 }
 
+// NewFromInt converts a int64 to Decimal.
+//
+// Example:
+//
+//     NewFromInt(123).String() // output: "123"
+//     NewFromInt(-10).String() // output: "-10"
+func NewFromInt(value int64) Decimal {
+	return Decimal{
+		value: big.NewInt(value),
+		exp:   0,
+	}
+}
+
+// NewFromInt32 converts a int32 to Decimal.
+//
+// Example:
+//
+//     NewFromInt(123).String() // output: "123"
+//     NewFromInt(-10).String() // output: "-10"
+func NewFromInt32(value int32) Decimal {
+	return Decimal{
+		value: big.NewInt(int64(value)),
+		exp:   0,
+	}
+}
+
 // NewFromBigInt returns a new Decimal from a big.Int, value * 10 ^ exp
 func NewFromBigInt(value *big.Int, exp int32) Decimal {
 	return Decimal{

decimal_test.go

@@ -477,6 +477,56 @@ func TestNewFromFloatWithExponent(t *testing.T) {
 	}
 }
 
+func TestNewFromInt(t *testing.T) {
+	tests := map[int64]string{
+		0:                   "0",
+		1:                   "1",
+		323412345:           "323412345",
+		9223372036854775807: "9223372036854775807",
+	}
+
+	// add negatives
+	for p, s := range tests {
+		if p > 0 {
+			tests[-p] = "-" + s
+		}
+	}
+
+	for input, s := range tests {
+		d := NewFromInt(input)
+		if d.String() != s {
+			t.Errorf("expected %s, got %s (%s, %d)",
+				s, d.String(),
+				d.value.String(), d.exp)
+		}
+	}
+}
+
+func TestNewFromInt32(t *testing.T) {
+	tests := map[int32]string{
+		0:          "0",
+		1:          "1",
+		323412345:  "323412345",
+		2147483647: "2147483647",
+	}
+
+	// add negatives
+	for p, s := range tests {
+		if p > 0 {
+			tests[-p] = "-" + s
+		}
+	}
+
+	for input, s := range tests {
+		d := NewFromInt32(input)
+		if d.String() != s {
+			t.Errorf("expected %s, got %s (%s, %d)",
+				s, d.String(),
+				d.value.String(), d.exp)
+		}
+	}
+}
+
 func TestNewFromBigIntWithExponent(t *testing.T) {
 	type Inp struct {
 		val *big.Int