This is very important when several Lua tactics are implemented in the
same Lua State object. In this case, even if we use the par
combinator, a Lua tactic will block the other Lua tactics running in
the same Lua State object.
With this commit, a Lua tactic can use yield to allow other tactics
in the same State object to execute.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
