Don't force the internal use of `RefCell<T>`, let the caller place that into
`MainThread<>` manually. This lets us remove the reference to `MainThread<>`
from the definition of `Screen` again and reduces the number of
`assert_is_main_thread()` calls.
This allows us to get the terminfo information without linking against curses.
That means we can get by without a bunch of awkward C-API trickery.
There is no global "cur_term" kept by a library for us that we need to invalidate.
Note that it still requires a "unhashed terminfo database", and I don't know how well it handles termcap.
I am not actually sure if there are systems that *can't* have terminfo, everything I looked at
has the ncurses terminfo available to install at least.
