There is no compiler error when using an invalid Decimal type.

**Describe the bug**
Users can create Decimal values with invalid BYTE_WIDTH, such as
```rust
let v = BasicDecimal::<1>::new(1, 1, &[1]);
```

**Expected behavior**
We need to add more restriction to the decimal type so that only `Decimal128` and `Decimal256` can be used.

**Additional context**