finally!

I propose we add `try .. finally! ...` to allow `finally` actions to be monadic.

The current computation-expression desugaring of `try .. finally ... ` allows only a `unit -> unit` action in the compensation branch.. However there are cases where an asynchronous or other monadic imperative action makes sense in exception compensation, e.g. using AsyncSeq:

```fsharp
asyncSeq {
try
do! Async.Sleep 2000;
yield UpdateItemsReceived [| { Text = "AsyncLoad1"; Description = "AsyncDescription1" } |]
do! Async.Sleep 2000;
yield UpdateItemsReceived [| { Text = "AsyncLoad2"; Description = "AsyncDescription2" } |]
do! Async.Sleep 2000;
finally!
yield UpdateItemsComplete
}

```

The current way of doing this in F# is some mess like this:

asyncSeq {
do! Async.Sleep 2000;
yield UpdateItemsReceived [| { Text = "AsyncLoad1"; Description = "AsyncDescription1" } |]
do! Async.Sleep 2000;
yield UpdateItemsReceived [| { Text = "AsyncLoad2"; Description = "AsyncDescription2" } |]
do! Async.Sleep 2000;
}
|> (fun a -> AsyncSeq.tryFinallyMonadic(a, fun () -> ...))

The same effect can also currently be achieved with a custom operation, something like this, but just supporting `finally!` looks more sensible:

asyncSeq {
try
do! Async.Sleep 2000;
yield UpdateItemsReceived [| { Text = "AsyncLoad1"; Description = "AsyncDescription1" } |]
do! Async.Sleep 2000;
yield UpdateItemsReceived [| { Text = "AsyncLoad2"; Description = "AsyncDescription2" } |]
do! Async.Sleep 2000;
yieldFinally UpdateItemsComplete
}

## Pros and Cons

The advantages of making this adjustment to F# are orthogonality/simplicity.

The disadvantages of making this adjustment to F# are cost.

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

## Related Items

* https://github.com/fsharp/fslang-suggestions/issues/579

Start by reading the proposed `finally!` syntax, the computation-expression desugaring described here, and related issue #579. No implementation files, tests, or compiler entry points are named, so identify the relevant language-design and compiler areas before proceeding. Done would require an accepted design and corresponding implementation and tests.

Written by the indexing model from the issue text.