[Arith] add SizeVar representing non-neg valued variable in a tensor shape (#4684)

[Arith] add SizeVar representing non-neg valued variable in a tensor shape (#4684)

* [arith] add ShapeVar representing non-neg valued variable in a tensor shape

* bounder remover; deal with div in int_set differently

* fix bounder_remover

* migrate unittest to use shape_var

* use tvm.shape_var in integration & relay tests

* add test case; fix Var register

* fix lint

* fix lint again

* add default ShapeVar visitor in Relay

* fix override

* fix ShapeVar visit bug

* revert IntervalSet for shape_var

* remove bound_remover

* remove is_var; use constructor for shapevar/var instead

* ShapeVar -> SizeVar; add constructor comments

* shape_var -> size_var in doc

* tindex -> size