* [arith] linear system and equation solver

Co-authored-by: Sergei Grechanik <sergei.grechanik+h@gmail.com>

* avoid constructing analyzer every time

* generate random test cases and address comments

Co-authored-by: Sergei Grechanik <sergei.grechanik@gmail.com>

* rename linear_system to int_constraints

* add comments and use random seed

* message for reporting failure with seed

* add SEqualReduce to IntConstraints; allow variables & ranges to be None

Co-authored-by: Sergei Grechanik <sergei.grechanik+h@gmail.com>
Co-authored-by: Sergei Grechanik <sergei.grechanik@gmail.com>