simp_arith
This PR deprecates the tactics `simp_arith`, `simp_arith!`, `simp_all_arith` and `simp_all_arith!`. Users can just use the `+arith` option.
simp +arith
This PR adds `simp +arith` for integers. It uses the new `grind` normalizer for linear integer arithmetic. We still need to implement support for dividing the coefficients by their GCD. It also fixes several bugs in the normalizer.
Offset.lean
Motivation: avoid the unfold and check idiom. This commit also minimize dependencies at `Offset.lean`. closes #2615