parking 1.0.5

Thread parking and unparking
Documentation
# Version 1.0.5

- Implement `Default` for `Parker`.

# Version 1.0.4

- Forbid unsafe code.

# Version 1.0.3

- Improved documentation.

# Version 1.0.2

- Remove all unsafe code.

# Version 1.0.1

- Explain `Parker::park()` better.

# Version 1.0.0

- Initial version.