Fix thread safety issue.
Fix thread safety issue.

ProgressDrawTarget was unsafe impl Sync which hid a true threading
issue: multiple threads could call Sender::send simultaneously, which is
not permitted by the mpsc API.

Wrapping Sender in a Mutex not only fixes the bug but also permits us to
remove the unsafe impls. The same type is the reason for the unsafe impl
of MultiProgress; however, in that case, the call to Sender::clone is
protected by holding the write lock on MultiProgress::state.

mitsuhiko/indicatif