utf8: let's update utf16_to_utf8() a bit

utf8: let's update utf16_to_utf8() a bit

Let's change utf16_to_utf8() prototype to refer to utf16 chars with char16_t rather than void

Let's not cast away a "const" needlessly.

Let's add a few comments.

Let's fix the calculations of the buffer size to allocate, and how long
to run the loop in case of uneven byte numbers