Promote div operation with div (not /)

[hersle]

This change uses div in the divrem function, rather than /.

This prevents integer divison from being promoted to floats, fixing the downstream issue JuliaSymbolics/Symbolics.jl#1083.

However, I see it breaks Multivariate gcd Rational{BigInt} tests, so I suppose it WIP now, at best.

Is this change viable to incorporate? Sorry, my knowledge of this library is very limited!