SomeAsync and value replacement #144

-- Not very nice. Allocates wrappers unnecessarily.
data SomeAsync = forall a. SomeAsync !(Async a)
cancelSomeAsyncs :: [SomeAsync] -> IO ()

-- Is a `some` dependency okay, or copying stuff from there?
cancelSomeAsyncs :: [Some Async] -> IO ()

-- Potentially expensive if there's a lot of `fmap`ping and not a lot of inlining.
cancelAsyncs [() <$ thisOne, () <$ thatOne]

-- Can we make <$ cheaper? Yes, but it makes `Async` bigger. Any we need
-- to unsafeCoerce with Any to allow Async to unbox (ugh).
data Async a = Async !ThreadId !(TMVar (Either SomeException Any)) (Any -> a)
a <$ Async tid mv fun = Async tid mv (const a)