Declare DecimalArray::length as a constant

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As for decimal 128 array, the value length is always 16 bytes, we should declare it as a constant.

Describe the solution you'd like
something like

impl DecimalArray {
    const VALUE_LEN: i32 = 16;
    ...
}

and remove the field DecimalArray::length

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A clear and concise description of any alternative solutions or features you've considered.

Additional context
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