Minor Refactoring to `NumberHelper#number_to_human`

* Use destructive `map` and `sort_by` to save extra object creation.
* Create `INVERTED_DECIMAL_UNITS.invert` constant instead of repeatedly doing `DECIMAL_UNITS.invert`
Merge pull request #12534 from vipulnsward/minor_number_helper_perfs

Minor Refactoring to `NumberHelper#number_to_human`