[#168] sdk/accounting: Refactor Decimal type

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/accounting/decimal_test.go

@@ -0,0 +1,25 @@
+package accounting
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDecimal_Value(t *testing.T) {
+	d := NewDecimal()
+
+	v := int64(3)
+	d.SetValue(v)
+
+	require.Equal(t, v, d.Value())
+}
+
+func TestDecimal_Precision(t *testing.T) {
+	d := NewDecimal()
+
+	p := uint32(3)
+	d.SetPrecision(p)
+
+	require.Equal(t, p, d.Precision())
+}