Move IterationCallbacks into their own file.

1. Merge TrustRegionLoggingCallback and LineSearchLoggingCallback
   into a single callback.
2. Move the callbacks into callback.h
3. Update SolverImpl to use the new callbacks.

Change-Id: I9e82173cf2b828d023d96c57d1cba17f4832aeae
4 files changed