Improve multiplication for decimal128_fast

[mborland]

Closes: #659

Provides a 22% speedup over develop. Likely still could be better.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.3%. Comparing base (cee3df5) to head (c011208).

Additional details and impacted files

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