Safe concurrency in D

This package tries to make using shared easier in D by locking and unlocking a mutex for the user.

It also aims to be @safe by using scope and DIP1000.

For now consult the example code in the only source file.

The Rust type is called Mutex but here it's called Shared. It's not a good name, but also calling it Mutex might have been confusing given core.sync.mutex.Mutex.