core: ensure all types from execute.h start with Exec

core: ensure all types from execute.h start with Exec

Until very recently all types defined by execute.h started with "Exec"
in the name. I think that was useful, since it made clear that the types
are associated with the ExecContext infrastructure. Let's hence restore
this.

(If we every move these types out of execute.h we should drop the "Exec"
prefix again. But today is not that day.)

No real code changes, just dumb renaming.