Improve dec64 multiplication

[mborland]

Closes: #653

Provides an ~40% speedup to decimal64 by replacing O(log(N)) comparisions with 1

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.5%. Comparing base (1cd6c70) to head (e57bff0).

Additional details and impacted files

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