[DPP-57] Provide versions of handle{Cast|Call}_ that work in the Process monad

[qnikst]

[Imported from JIRA. Reported by Tim Watson [Administrator] @hyperthunk) as DPP-57 on 2013-01-14 22:12:01]
Consider these two cast handlers:

handleCastIf_ (\(c :: String, _ :: Delay) -> c == "timeout")
                            (\("timeout", Delay d) -> timeoutAfter_ d),
handleCast    (\s' ("ping", pid :: ProcessId) ->
                                 send pid "pong" >> continue s')

There's no great reason why we shouldn't be able to skip the state in the latter as we do with the former, but the continue_ function returns (s -> Process (ProcessAction s)) which means it can't be used in monadic code like this.