divrem broken in packages using AbstractAlgebra

[thofma]

@wbhart @heiderich
Before #252 :

julia> using Nemo

julia> Qx, x  = PolynomialRing(FlintQQ, "x");

julia> K, a = NumberField(x^3 + 3x + 1, "a")
(Number field over Rational Field with defining polynomial x^3+3*x+1, a)

julia> divrem(a, a)
(1, 0)

After #252

julia> divrem(a, a)
ERROR: MethodError: no method matching rem(::nf_elem, ::nf_elem)
Closest candidates are:
  rem(::Any, ::Any, ::RoundingMode{:ToZero}) at math.jl:739
  rem(::Any, ::Any, ::RoundingMode{:Down}) at math.jl:740
  rem(::Any, ::Any, ::RoundingMode{:Up}) at math.jl:741

[wbhart]

Yeah, I thought it would be broken. We must overload Base.divrem everywhere (except for Julia types), not divrem. Otherwise it falls back to a generic method which calls rem. And of course that is not defined in this case.