Inconsistent modulo / remainder behaviour

Different backends act differently given the following file:
```scala
def main() = {
println(7.mod(3)) // 7 % 3
println(7.mod(neg(3))) // 7 % -3
println(neg(7).mod(3)) // -7 % 3
println(neg(7).mod(neg(3))) // -7 % -3
}
```

JS, LLVM:
```
1
1
-1
-1
```

but on Chez:
```
1
-2
2
-1
```

because Chez actually does `(modulo x y)`, but other backends do `(remainder x y)`.

I think the proper solution here would be to make Chez consistent with other backends (`s/modulo/remainder` in the code), then rename `mod` to `rem`, and add an _actual_ `mod`ulo to complement it, much like Haskell does.