Revert "Merge pull request #200 from sigp/new-structure"

This reverts commit d7a3545be1, reversing
changes made to 1da06c156c.

eth2/utils/slot_clock/src/lib.rs

@@ -0,0 +1,12 @@
+mod system_time_slot_clock;
+mod testing_slot_clock;
+
+pub use crate::system_time_slot_clock::{Error as SystemTimeSlotClockError, SystemTimeSlotClock};
+pub use crate::testing_slot_clock::{Error as TestingSlotClockError, TestingSlotClock};
+pub use types::Slot;
+
+pub trait SlotClock: Send + Sync {
+    type Error;
+
+    fn present_slot(&self) -> Result<Option<Slot>, Self::Error>;
+}