unicode: String API is const byte*.

We still have that char vs byte dichotomy, but majority of string operations
now use byte.
diff --git a/py/misc.h b/py/misc.h
index fd54147..97e16c2 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -88,8 +88,8 @@
 
 typedef int unichar; // TODO
 
-unichar utf8_get_char(const char *s);
-char *utf8_next_char(const char *s);
+unichar utf8_get_char(const byte *s);
+const byte *utf8_next_char(const byte *s);
 
 bool unichar_isspace(unichar c);
 bool unichar_isalpha(unichar c);