CallbackSigner is unsound

[kornelski]

It implements Send and Sync, but allows use of non-Send non-Sync callback functions. This can be used to create a data race in safe Rust code. CallbackFunc should be defined as:

pub type CallbackFunc = dyn Fn(*const (), &[u8]) -> std::result::Result<Vec<u8>, Error> + Send + Sync;

[gpeacock]

Thank you for this! fixed in b190464