Introduce an example of tactic development using the DSL. (#141)

lisa-examples/src/main/scala/Example.scala

...-utils/src/main/scala/lisa/prooflib/BasicStepTactic.scala

lisa-utils/src/main/scala/lisa/prooflib/ProofsHelpers.scala

...ils/src/main/scala/lisa/prooflib/SimpleDeducedSteps.scala

lisa-utils/src/main/scala/lisa/prooflib/WithTheorems.scala

lisa-utils/src/main/scala/lisa/utils/KernelHelpers.scala

lisa-utils/src/main/scala/lisa/utils/parsing/Parser.scala

.../main/scala/lisa/utils/unification/UnificationUtils.scala

.../scala/lisa/automation/kernel/OLPropositionalSolver.scala

...la/lisa/automation/kernel/SimplePropositionalSolver.scala

src/main/scala/lisa/automation/kernel/SimpleSimplifier.scala