Seal the decimal type.

[HaoYang670]

Which issue does this PR close?

Closes #2440.

Rationale for this change

The current implementation of BasicDecimal cannot restrict the BYTE_WIDTH to be 16 or 32, which means that you can create an invalid decimal value like this

let v = BasicDecimal::<1>::new(1, 1, &[1]);

What changes are included in this PR?

We create a new public trait ValidDecimal and implement the trait for Decimal128 and Decimal256. The idea is from https://users.rust-lang.org/t/how-to-seal-the-const-generic/77947/2.
Because of the orphan rule, downstream users cannot implement ValidDecimal for other BasicDecimal types.

Are there any user-facing changes?

No breaking.

[HaoYang670]

Hi @tustvold, Could you please help review?

[HaoYang670]

@viirya @liukun4515 Could you please help to review?

[tustvold]

As #2001 hasn't been released yet, I think we have an opportunity to revisit this prior to the next release #2382 (comment)

In particular using a const generic and then constraining it in this way feels a bit derived. Perhaps we could instead have

pub trait DecimalType {
    const BYTE_LENGTH: usize;
    const MAX_PRECISION: usize;
    const MAX_SCALE: usize;
    const TYPE_CONSTRUCTOR: fn(usize, usize) -> DataType;
    const DEFAULT_TYPE: DataType;
}

pub struct Decimal128Type {}

pub struct Decimal256Type {}

pub struct DecimalArray<T: DecimalType>

[HaoYang670]

Hi @tustvold, do you mean that you want to revert #2383?

[tustvold]

Hi @tustvold, do you mean that you want to revert #2383?

I was more wondering if we could build upon it, to polish off some rough edges. Perhaps you could take a look at #2477 and let me know what you think?