* src/vm/utf8.c, src/vm/utf8.h (utf_bytes): New function.

* src/vm/utf8.c, src/vm/utf8.h (utf_bytes): New function.
Use this function to query the number of octets of a utf string.
This function will become the correct replacement for the old
utf_strlen in most places. The calling code will have to be
reviewed for that, however, and it will also require fixing
utf_strcat, utf_sprint, and so on, to do the right thing.