Don't import `_fill_lmul!`

[jishnub]

This isn't used anywhere in this package

[codecov[bot]]

Codecov Report

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

Comparison is base (9ce85e3) 88.23% compared to head (ed7aa3c) 88.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #178 +/- ## ======================================= Coverage 88.23% 88.23% ======================================= Files 11 11 Lines 1190 1190 ======================================= Hits 1050 1050 Misses 140 140 ```

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