This commit also removes the laziness for accessing symbols, since they
are already in the scope at the time the plugin is loaded.

Temporary trick to enforce encoding in presence of algebraic types,
even if they are not polymorphic

probably not efficient because not memoized (Claude needs help for that)

this allows to provide the time limit on the command of CVC4, and perhaps Z3 later.

Conflicts:
	examples/bts/execute.mlw

(reported by mario.alvarez739@gmail.com)

slight improvements of CVC4 driver, preparing the support for the "steps" feature and the counter-examples