This function converts a single wide character into its multibyte
representation (UTF-8 in our case). It is called from libc++'s
`std::basic_ostream<wchar_t>::flush`, which gets called at program exit
from a global destructor in order to flush `std::wcout`.
This introduces the UnicodeUtils file, which contains helper functions
related to Unicode. This is in contrast to StringUtils, whose functions
are not directly related to Unicode and are, in theory,
encoding-agnostic.
