feat(opensmt): add abs function
fix(opensmt/parsers/smt2/CMakeLists.txt): dependency on smt2parser/lexer
fix(opensmt/parsers/smt2/CMakeLists.txt): suppress sign-compare warning
feat(opensmt/egraph/EgraphStore.C): remove sort check in mkEq

For now, we remove the requirement that

    "sort(LHS) = sort(RHS)"

. We still need to check LHS and RHS are Real/Int (or Int/Real).
tests(nra): add abs tests