refactor(input): don't use a ring for input

Since paste data is handled via a separate channel, the data processed via `input_buffer` is typically just explicit keys as typed in by the user. Therefore it should be fine to use `memmove()` to always put the remaining data in front when refilling the buffer.

src/nvim/api/vim.c

@@ -313,7 +313,7 @@ void nvim_feedkeys(String keys, String mode, Boolean escape_ks)
     keys_esc = keys.data;
   }
   if (lowlevel) {
-    input_enqueue_raw(cstr_as_string(keys_esc));
+    input_enqueue_raw(keys_esc, strlen(keys_esc));
   } else {
     ins_typebuf(keys_esc, (remap ? REMAP_YES : REMAP_NONE),
                 insert ? 0 : typebuf.tb_len, !typed, false);