This notably adds a `MonadBaseControl` instance for the `REPL` monad so
that we can catch asynchronous exceptions in the middle of `REPL`
computations. Since this is a generalization of Haskeline's exception
support, that orphan instance for the console executable is now in terms
of this new instance.
This removes the `self-contained` flag, since it is fine for all builds
to have the baked-in prelude as a last resort. Tinkerers can still
change the prelude as long as it's in the search path.
Also removes some unnecessary extra prelude loading by the Cryptol
server by means of a new command
