Termion backend is slow

When running the `lorem` example with the termion backend in debug mode, a serious flickering can be observed. Running in release mode seems to solve this. Running in debug mode with the ncurses backend is also fine.
It may be caused by a slow backend. All the calls to `print!` have to lock individually the standard output.

Note: to be clear, this issue is about the wrapper code in the cursive repository, not about termion itself.