Limit use of `newObstack` to implementation of `withRegion` functions

[Josh Berdine]

Also rename newObstack to e.g. unsafeNewObstack.

Care must be taken when creating a new obstack since if mutable values from the old obstack are used by computations run in the new obstack, it is possible to create old-to-young pointers, and the runtime system does not handle them safely.

See discussion: #126