feat!: remove Progress: 'static requirement.

This requirement can be added where used and where needed, and
originally snuck in because it was easier and `Progress` implementations
typically are `'static` as well.

However, that requirement made it impossible to implement `Progoress`
for `&mut T where T: Progress`.

src/traits.rs

@@ -2,7 +2,7 @@ use crate::{messages::MessageLevel, progress, Unit};
 use std::time::Instant;
 
 /// A trait for describing hierarchical process.
-pub trait Progress: Send + 'static {
+pub trait Progress: Send {
     /// The type of progress returned by [`add_child()`][Progress::add_child()].
     type SubProgress: Progress;