MonadFail instance (feature request)

It would be very handy to have a `MonadFail` instance that `retry`s. As a minimal motivating example, consider:

pop :: TVar [a] -> STM a
pop v = do
a:as <- readTVar v
writeTVar v as
return a

With a `MonadFail` instance, this is a very convenient way of writing a transaction that waits until there's a value available in the stack.

More generally, such an instance would allow one to write a transaction as if all your favorite patterns matched, and the transaction would then efficiently block until it's so.

Start by reviewing the existing STM retry behavior and the MonadFail instance or related interfaces, using the pop example in the issue as the motivating case. Determine the intended semantics for pattern-match failure, blocking, and transaction retry; the issue is complete only when those semantics and corresponding tests are defined and implemented.

Written by the indexing model from the issue text.