Merge t_graph definition into mshift.h

- Remove types/graph.h, and merge its contents into mshift.h.
  This is a natural place for the structure, as the header already
  contains most functions that operate on the graph.
- Make t_graph forward-declarable, and use that in headers that
  previously got t_graph from typedefs.h.
- Add some missing mshift.h includes to source files that now need it.

Some additional cleanup of mshift.h dependencies will be done in
subsequent commits, when the header itself is moved.

Change-Id: Ia8c5817c11474c857a6845810e5e7f5a75f1c47c