add method to snapshot TBQueue without flushing

Currently there is no way to efficiently snapshot the state of TBQueue without flushing it and rewriting (and neither there is a way to efficiently create TBQueue from the list). What is required to do to a snapshot now:

```haskell
snapshotTBQueue :: TBQueue a -> STM [a]
snapshotTBQueue q = do
xs <- flushTBQueue q
mapM_ (writeTBQueue q) xs
pure xs
```

But snapshot is a part of flushTBQueue, and if TBQueue constructor was exported it could have been implemented outside in this way:

```haskell
snapshotTBQueue :: TBQueue a -> STM [a]
snapshotTBQueue (TBQueue _ read _ write _) = do
xs <- readTVar read
ys <- readTVar write
return $ if null xs && null is then [] else xs ++ reverse ys
```

The use case for snapshot is dumping a state of the queue(s) to the hard-drive, so it can be efficiently snapshotted before dump starts (so that the state is consistent).

Could we add snapshot to STM? Alternatively, could we export TBQueue constructor?