Export `tlsAppData` which creates `AppData` from a TLS `Context`.

This will allow users to build a TLS `Context` in whatever way they wish
and then make conduit out of it.

Simple client example:
```haskell
import Network.Simple.TCP.TLS

myapp :: Application IO
...
main = do
    cset <- getDefaultClientSettings
    connect cset "host" "port" $
        (\(ctx, addr) -> myapp $ tlsAppData ctx addr Nothing)
```

I also replaced `let src = ... >> src in src` by `forever $ ...`, which
gets inlined into the very same thing.