ghci: make the Pipe type opaque

This commit makes the Pipe type in ghci opaque, and introduce the
mkPipeFromHandles constructor for creating a Pipe from a pair of
Handles. Pipe is now just a pair of receiver/sender continuations
under the hood. This allows a Pipe to be potentially backed by other
IPC mechanisms (e.g. WebSockets) which is essential for wasm ghci
browser mode.