msgpack: add string() for decimal

Added a benchmark, which shows that the code is optimized two or more times for string conversion than the code from the library.

Added #322

Author: babyTsakhes

decimal/decimal_test.go

@@ -893,3 +893,27 @@ func TestLargeNumberStringOptimized(t *testing.T) {
 
 	t.Logf("Large number handled via fallback: %s", optimized)
 }
+
+func TestDecimalTrailingZeros(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{"100.00", "100.00"},
+		{"0.00", "0.00"},
+		{"0.000", "0.000"},
+		{"1.000", "1.000"},
+		{"123.4500", "123.4500"},
+		{"0.00100", "0.00100"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			dec := MustMakeDecimal(tt.input)
+			result := dec.StringOptimized()
+			if result != tt.expected {
+				t.Errorf("For %s: expected %s, got %s", tt.input, tt.expected, result)
+			}
+		})
+	}
+}