Allow to pad an UTF-8 or other multibyte string

Summary:
This method is imported from rKGF, where it was prepared for Auth Grove.

See also b31a1e77d0ea and a7e03a653500.

Author credit: Ronald Ulysses Swanson aka Wes

Ref T1395.

Test Plan: Unit tests are included.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1395

Differential Revision: https://devcentral.nasqueron.org/D1601