feat(library/unifier): add unifier.max_steps unifier.use_exceptions options

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): add imitation step for macros

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/constraint): add 'delayed' flag to choice constraints

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/kernel_bindings): add mk_choice_cnstr to Lua API

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