RFC: make utf16 to/from utf8 functions private to the core

These functions are not listed in the public API, though they are 
accessible from anywhere.  They are not used in CPAN.

I propose to mark them as core-only, so they can't be used (without 
skulduggery) outside it.  The reason is that I think a better API should 
be created.  The problem is that the caller now is supposed to know the 
worst case buffer size, preallocate that, and the function blindly 
assumes this has been done, so if it hasn't out-of-bounds writes are 
possible.   Worse, the documentation, until a very recent commit I made, 
specified too small a size, one which would overflow in the worst case 
(though only on EBCDIC platforms).  (That documentation, since these 
aren't in the public API) is in the form of comments at the function source.

It is a potential security hole to allow this.  That's why we use 
strlcpy() instead of strcpy(), and have changed the other Unicode 
functions to pass in a length.

• RFC: make utf16 to/from utf8 functions private to the core by Karl Williamson
• Re: RFC: make utf16 to/from utf8 functions private to the core by Andy Lester
• Re: RFC: make utf16 to/from utf8 functions private to the core by Sawyer X