fix(kernel/formatter): add missing parentheses

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/type_checker): simplify type checker API, and remove add_cnstr_fn

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix (library/unifier): occurs_context_check

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix (library/unifier): occurs_context_check