Add "ThrowableCallable" a Callable that allows a Throwable to be thrown

@yborovikov for review. If you have a better name for ThrowableCallable (preferably shorter), I'm happy to take it.

Most other portions of ParSeq allow a Throwable to be raised. We can't
provide this functionality with Callable because it is limited to
raising Exceptions. The new ThrowableCallable class addresses this.