Add Euclidean-based div and mod

[dvdvgt]

Since this is based on effekt definitions and not FFI, this has the added side-effect of also unifying the behavior across all backends.

[dvdvgt]

All backends except LLVM used to use floor-based integer division prior to this PR such that 8 / -3 = -3 and not 8 / -3 = -2. Thus, I changed the infixDiv implementations for JS and Chez to make it consistent with LLVM

[jiribenes]

Stupid question: Will this work on LLVM where Int is i64 as opposed to f64 (~i32 for the purposes of bin ops) like in JS?

[dvdvgt]

It appears to be working and yielding consistent results with the remaining backends. I'll do some further testing, though. Good point, thanks!

[phischu]

Please add a fast implementation of mod for llvm and other platforms where this is possible:

extern def mod(x: Int, y: Int): Int at {} =
  llvm " ... "
  default { current code }