chore(library/tactic): remove dead code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(library/tactic): update goal objects

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(library/tactic): remove unnecessary file

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/tactic/cex_builder): simplify cex_builder

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/tactic/goal): use local names for hypotheses

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/tactic/proof_builder): simplify proof builder

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/tactic/proof_state): simplify proof state

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>