Add Copy func (#123)

* Add Copy method
Co-authored-by: habuvo <Sin90is0>

decimal.go

@@ -1886,3 +1886,14 @@ func (d Decimal) Tan() Decimal {
 	}
 	return y
 }
+
+// Copy makes instance of d with same value and different pointer.
+func (d Decimal) Copy() Decimal {
+	d.ensureInitialized()
+	return Decimal{
+		value: &(*d.value),
+		exp:   d.exp,
+	}
+}
+
+

decimal_test.go

@@ -3288,3 +3288,21 @@ func ExampleNewFromFloat() {
 	//0.123123123123123
 	//-10000000000000
 }
+
+//For Copy
+func TestCopy(t *testing.T) {
+	origin := New(1, 0)
+	cpy := origin.Copy()
+
+	if cpy.Cmp(origin) != 0 {
+		t.Error("copy and origin not equal")
+	}
+
+	//change value
+	cpy = cpy.Add(New(1, 0))
+
+	if cpy.Cmp(origin) == 0 {
+		t.Error("copy and origin are equal but expected not")
+	}
+
+}