Prevent fmt::Arguments from being shared across threads
Fixes#45197
This is a **breaking change**! Without doing this it's very easy to create race conditions.
There's probably a way to do this without breaking valid use cases, but it would require quite an overhaul of the formatting machinery.
On cases of malformed format strings where a `{` hasn't been properly
escaped, like `println!("{");`, present a note explaining how to escape
the `{` char.
