Deprecates functions that use wall-clock time, so that code must be explicit about what time source is used.

Idiomatic usage is:

          open! Require_explicit_time_source

or, in an import.ml:

          include Require_explicit_time_source

`module Clock_ns : Async_kernel__.Clock_intf.Clock_deprecated with module Time := Async_kernel__.Import.Time_ns`
`module Time_ns : sig ... end`
`module Scheduler : sig ... end`
`val at : Time_ns.t ‑> unit Async_kernel.Deferred.t`

- Deprecated [since 2016-02] Use [Time_source]

`val after : Time_ns.Span.t ‑> unit Async_kernel.Deferred.t`

- Deprecated [since 2016-02] Use [Time_source]

`val every : ?⁠start:unit Async_kernel.Deferred.t ‑> ?⁠stop:unit Async_kernel.Deferred.t ‑> ?⁠continue_on_error:bool ‑> Time_ns.Span.t ‑> (unit ‑> unit) ‑> unit`

- Deprecated [since 2016-02] Use [Time_source]

`val with_timeout : Time_ns.Span.t ‑> 'a Async_kernel.Deferred.t ‑> [ `Timeout | `Result of 'a ] Async_kernel.Deferred.t`

- Deprecated [since 2016-02] Use [Time_source]