[CM] Unicode support

[bil at ccrma.Stanford.EDU]

> We just need to send a Unicode string across, from s7 to C to Swift.

I thought from your example that you wanted to see a filled
square.  If you want to encode a sequence of utf8 codepoints
(if that is the correct terminology), I think you can use
utf8proc_map or maybe utf8proc_encode_char (on each one),
filling a byte-vector, then pass that to C -- the byte-vector
data is a C array of bytes, so it can be used directly (see
s7_byte_vector_elements).  As I said before, I haven't had
time (or need) to learn much about Unicode, but I wrote
libutf8proc.scm because at that time the Julia language was
using it to implement its utf8 stuff.