vim-patch:8.1.0743: giving error messages is not flexible

Problem:    Giving error messages is not flexible.
Solution:   Add semsg().  Change argument from "char_u *" to "char *", also
            for msg() and get rid of most MSG macros. (Ozaki Kiichi, closes
            vim/vim#3302)  Also make emsg() accept a "char *" argument.  Get rid of
            an enormous number of type casts.
f9e3e09fdc

src/nvim/api/vim.c

@@ -1724,7 +1724,7 @@ static void write_msg(String message, bool to_err)
 #define PUSH_CHAR(i, pos, line_buf, msg) \
   if (message.data[i] == NL || pos == LINE_BUFFER_SIZE - 1) { \
     line_buf[pos] = NUL; \
-    msg((char_u *)line_buf); \
+    msg(line_buf); \
     pos = 0; \
     continue; \
   } \