Step 1.8: Thread safety

[ziimakc]

Resolves Step 1.8

Task

Implement the following types, which meet conditions:

1. OnlySync is Sync, but !Send.
2. OnlySend is Send, but !Sync.
3. SyncAndSend is both Sync and Send.
4. NotSyncNotSend is both !Sync and !Send.

All inner details of implementation are on your choice.

Play with these types from multiple threads to see how compile time [fearless concurrency][2] works in practice.

Solution

Complete task using existing Synd/Sync types to avoid nightly rust (negative impl). Add a few examples.