Fix build_rule

[Stevengre]

During the verification of automatic generated kclaim from summarization, current build_rule may lead to Could not determine sort of term error because the introduction of new variables in build_rule.

This PR tries the best to keep the sort of variables during their corresponding var introduction and keep its default behavior.

[ehildenb]

@Stevengre please add a test of this behavior where appropriate. There is a testing harness for build_rule here:

k/pyk/src/tests/unit/test_cterm.py

Line 125 in 4e07d71

@pytest.mark.parametrize('lhs,rhs,keep_vars,expected', BUILD_RULE_TEST_DATA, ids=count())

, which you can add to. Make sure that your test is failing before your change, and passing after your change.