For files that required flow.h, and only got it through actorcompiler.h,
their version of flow.h would have the actorcompiler #defines defined.
Then, if it included a STL/boost file, the same breakage would result.
This needs to not happen, so the include of flow.h in actorcompiler.h
was removed.
For some reason, clang thinks that the #ifndef's aren't matched by
#endif's if we #include "flow.h" in the middle. Regardless, it's
better for compile time slightly to forward declare, so let's just do
that anyway.
It also maybe gets us away from a weird
actorcompiler.h -> flow.h -> genericactor.actor.h -> actorcompiler.h
circular include.
Jingyu Zhou