v8.0.0

Changes:

1. Reexport chrono and time crates (@deniskolodin).
2. Conditional compilation and io module refactoring (@deniskolodin).
3. socket, pipe and uuid features removed.
4. Native ssl support on OS X (via security framework).
5. Fix unicode handling in parse_named_params.
6. ssl feature is now opt-in.
7. GenericConnection trait introduced (@alex-gulyas).
8. Traits reexports moved to mysql::prelude module.