feat(library/fo_unify): unify heterogeneous - homogeneous equality

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): allow the user to use a theorem/axiom name as an argument for the apply tactic command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic): use _tac suffix instead of _tactic like Isabelle

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): add assumption command, and allow Lean expressions (proof terms) to be used with apply tactic command

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