Merge the forward and backward chainer into a mixed chainer

[ngeiswei]

Here are some vague guidelines for implementing the mixed chainer. Should be refined as we'll progress into the task.

1. Forward and backward chaining often need to occur over the same inference tree in order to construct the desire inference tree.
2. Providing a target may not always mean that the best chaining strategy should be backward, likewise providing sources may not always mean that best chaining strategy should be forward, these decisions should not be hardwired, and instead be delegated to control policies (possibly handwritten by default).
3. The mixed chainer should be able to take input intermediary sources/targets, not just end sources/targets.
4. It is expected as well that different parts of an inference tree could be evolved independently then stick together (using unification, just like a rule is stuck to an inference tree).