This PR implements a basic asynchronous timer API on top of the libuv
work.

It purposely puts this into `Std.Internal` as we might still have to
change the API as we continue develop of the async library across
releases so I would only like to stabilize it once we are certain this
is a fine API.

A few additional notes:
- we currently do not implement a bind operator on `AsyncTask` on
purpose as `Task.bind` on `Task.pure` is a non trivial operation and
users should be aware of it. Furthermore there is the consideration that
as they will have to bind on both `IO` and `AsyncTask` we might want to
make potential task points explicit in the syntax (did somebody say
`await`?).
- the API generally takes inspiration from
https://docs.rs/tokio/latest/tokio/time/index.html, though it has to
adapt as Rust's and Lean's asynchronity concepts are sufficiently
different.

Stacked on top of #6219.

This PR implements a basic asynchronous timer API on top of the libuv
work.

It purposely puts this into `Std.Internal` as we might still have to
change the API as we continue develop of the async library across
releases so I would only like to stabilize it once we are certain this
is a fine API.

A few additional notes:
- we currently do not implement a bind operator on `AsyncTask` on
purpose as `Task.bind` on `Task.pure` is a non trivial operation and
users should be aware of it. Furthermore there is the consideration that
as they will have to bind on both `IO` and `AsyncTask` we might want to
make potential task points explicit in the syntax (did somebody say
`await`?).
- the API generally takes inspiration from
https://docs.rs/tokio/latest/tokio/time/index.html, though it has to
adapt as Rust's and Lean's asynchronity concepts are sufficiently
different.

Stacked on top of #6219.

This PR implements a basic asynchronous timer API on top of the libuv
work.

It purposely puts this into `Std.Internal` as we might still have to
change the API as we continue develop of the async library across
releases so I would only like to stabilize it once we are certain this
is a fine API.

A few additional notes:
- we currently do not implement a bind operator on `AsyncTask` on
purpose as `Task.bind` on `Task.pure` is a non trivial operation and
users should be aware of it. Furthermore there is the consideration that
as they will have to bind on both `IO` and `AsyncTask` we might want to
make potential task points explicit in the syntax (did somebody say
`await`?).
- the API generally takes inspiration from
https://docs.rs/tokio/latest/tokio/time/index.html, though it has to
adapt as Rust's and Lean's asynchronity concepts are sufficiently
different.

Stacked on top of #6219.