Fix div/divrem

[wbhart]

This makes the internal definition of div/divrem in Nemo agree with the internal definition used by AbstractAlgebra (and with Flint).

This allows generic code to run correctly over Nemo.fmpz (e.g. the Generic.Modules test that I added as a regression test).

Fixes #842
Fixes #743 (because it was annoying me)

N.B. This is breaking and should not be merged until we have checked that no other packages break. In any event, at least Hecke.jl and Oscar.jl (and maybe Singular.jl) should import Nemo.div and Nemo.divrem instead of Base.div and Base.divrem, which will likely cause breakage.

I will check this out as soon as possible.

[thofma]

Does this mean Base.div(::fmpz, ::fmpz) and Nemo.div(::fmpz, ::fmpz) are different? So in Nemo and in the REPL the behaviors are different? This makes working with the only useful integer type that we have more cumbersome, just to cope with some questionable design desicions for BigInt, which no one should use anyway.

[thofma]

Let's continue this at #842.

[wbhart]

It's already different in AbstractAlgebra.

I have opened many tickets discussing possible solutions to this problem. There aren't really any options left.

[thofma]

I will make a new patch release with the new flint version (once it is merged upstream) and then we can merge this here.