Fix #381: Better API for preoptimizer and optimizer.

The API is now focused on the content needed for the
optimizers to do their job, rather than on a file system-like
architecture.
Restore continuations on 2.10.x, remove on 2.11.x.

In the past, the continuations library was built alongside
the scalalib. This commit restores this for 2.10.

However, it also removes definitively the continuations
library from builds for 2.11.x, as it is out-sourced in a
separate repo anyway in Scala itself.
Build for 2.10.4.

And remove build from 2.10.4-RC3.
Merge pull request #383 from sjrd/better-opt-api

Fix #381: Better API for preoptimizer and optimizer.
Merge pull request #387 from sjrd/build-2.10.4-and-2.11.0-RC3

Build for 2.10.4