Allow unsized types for Mutex?

[alekratz]

I'm trying to use a dyn Trait with a Mutex while moving from std::sync::Mutex to async_std::sync::Mutex. Is it possible for the Mutex in this crate to have T: ?Sized? The the std version allows for ?Sized types, but I don't know if this would be an issue for async_std.

[yoshuawuyts]

@alekratz o/ thanks for raising! I'd love for us to take ?sized, but am also unsure if that's possible. Currently nothing in async-std can take ?sized so my guess is that it's tricky, if not outright impossible.

@nbdd0121 maybe you have a better perspective on this?

[yoshuawuyts]

Closed in #625. This will be part of the next release (probably 1.4.0), due later this week. Thanks!