feat(library/tactic): execute Lua tactics using coroutines

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>