Plan: native Decimal{32|64|128|256} support

[DarkWanderer]

Target

Eliminate use of BigInteger as it is a slow heap type

Requirements

• Types must be blittable
• Scale information must be contained in type, not in memory, if possible
• Minimize code duplication where possible

Other notes

• Int128/Int256 types created for this can also be used for native support of respective ClickHouse types