This changes all the execute_graph() interfaces so that instead of
mutating a std::set of vertices, they accept an initial flat_set of
states and return a resultant flat_set of states after execution.
(Note that internally execute_graph() still uses bitsets)
This is both faster and more flexible.
We were using intermediate values int he enum and casting back and forth
with a u32; it is cleaner to just use a u32 and define some special
values.
Silences ICC warning #188: enumerated type mixed with another type.
