refactor: Harmonizing decimal128 and decimal256 into the same type

[forsaken628]

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Summary

Not distinguishing between decimal128 and decimal256 in terms of data type allows us to store decimal with a more flexible underlying type. using a smaller underlying type gives better performance.

Breaking Change

The previous implementation of hash's function overload on decimal was incorrect, and would actually convert decimal to float and then compute hash instead of directly. So this PR changes the result of all decimal hash.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is