This allows arbitrary char16 arrays to round-trip through UTF-8 without losing information, even if the char16 arrays are not valid UTF-16.

This is necessary e.g. for filesystem manipulation on Windows, where filenames contain 16-bit characters but valid UTF-16 is not enforced.

Invalid UTF-16 represented in UTF-8 is affectionately known as WTF-8: http://simonsapin.github.io/wtf-8/