…nfirmation prompt (which will come next)

… the Term type.

Instead, crossterm uses just a io::Write trait object.
Doesn't compile yet; many terminal methods are still missing.

In the next changes, I'll slowly add them to TermThemeRenderer instead
or replace calls by their crossterm equivalent.

(The buffered example is not adjusted yet; this will happen in the very end.)

The password prompt needs it, and it allows removing the Term accessor.

They are superfluous, now that TermThemeRenderer::clear also clears the current line.

This required adding another few helper methods to theme.rs,
but apart from that went quite smoothly.

The result is essentially untested; that's for later.

…ing errors remaining.

General things of note
- As crossterm operates on any io::Write trait object instead of
a dedicated Term struct, we use the static terminal::size()
method to obtain the terminal size, which returns an option.
- We conciously only support clearing an u16 number of lines, since that's
crossterm only allows moving the cursor by that many lines.
- We conciously manually clear the last N lines instead of the entire terminal
since the confirmation prompt expects it so.

This change was manually tested, by running the `confirm` example with
various inputs. The confirmation prompt behaves the same as before this change.

- create a new constant for the default terminal size,
as that's now used in three different places
- make clear_last_lines a static method, operating on any dyn io::Write,
now just a TermThemeRenderer -- paging also needs it.
- also extract clear_current_line to that level

…tance.

Use an Arc<Mutex<dyn io::Write>> instead of a &mut dyn io::Write.
- an arc allows sharing the terminal, a mutex allows mutating it while shared

To avoid lots of boilerplate with .lock().unwrap(), introduce a wrapper type.

Make the remaining prompts compile; almost all examples compile now.

The sole exception is the buffered example, which uses console's buffered_stderr type.

- {show,hide}_cursor can be removed now,
- flush() can be reduced to one single occurrence.

These were needed before we wrapped our terminal in an Arc.

Replaces lots of boilerplate with Term::new(Arc::new(Mutex::new(_))).