Add async shorthands

[granicz]

I propose to add the following shorthands to make working with reactive vars/views easier (which are best handled asynchronously, instead of reading their values directly):

1. Var.GetAsync, as a counterpart to View.GetAsync:

   type Var<'T> with
       member this.GetAsync =
           this.View |> View.GetAsync

2. Allowing Var<'T> and View<'T> to participate in async let! expressions:

   [<AutoOpen;JavaScript>]
   module Extensions =
       type AsyncBuilder with
           member this.Bind(var: Var<'T>, continuation: 'T -> Async<'U>) : Async<'U> =
               var.View |> View.GetAsync |> fun res -> async.Bind(res, continuation)
           member this.Bind(view: View<'T>, continuation: 'T -> Async<'U>) : Async<'U> =
               view |> View.GetAsync |> fun res -> async.Bind(res, continuation)

The latter would be especially useful, and would allow the following:

MainTemplate.ContactForm()
    .OnSend(fun e ->
        async {
            // Instead of reading e.Vars.Name.View |> View.GetAsync,
            // a much nicer way would be:
            let! name = e.Vars.Name
            // ... and same shorthand for views:
            let! email = e.Vars.Email.View
...