Disables a state.
This calls the state's fini-state
method, and then deactivates
all of the state's clauses.
The Main
state, which contains all of the class' toplevel forms, can't be disabled.
If the state is already inactive, it's an error.
If the state has any active child states, they will automatically be disabled before the parent state is disabled.
See also: @disab!
, init-state