Convert timestamp to unit type wrapper (#8333)

The timestamp inherent type was up to now just a simple `u64`. This
worked, but doesn't give you that much guarantees at compile time about
the type. This pr changes that by converting this type to a unit type
wrapper, similar to what we have done for `Slot`.

This is required for some future pr that touches quite a lot of the
inherents stuff :)

Besides this unit wrapper type, this pr also moves the `OnTimestampSet`
trait to `frame_support::traits`.

substrate/primitives/std/with_std.rs

@@ -37,6 +37,7 @@ pub use std::sync;
 pub use std::result;
 pub use std::slice;
 pub use std::str;
+pub use core::time;
 pub use std::vec;
 
 pub mod collections {

substrate/primitives/std/without_std.rs

@@ -39,6 +39,7 @@ pub use core::result;
 pub use core::slice;
 // Allow interpreting vectors of bytes as strings, but not constructing them.
 pub use core::str;
+pub use core::time;
 // We are trying to avoid certain things here, such as `core::string`
 // (if you need `String` you are probably doing something wrong, since
 // runtime doesn't require anything human readable).