Make digits for negative ZZRingElem consistent

[joschmitt]

The digits function behaves inconsistent with julia integer types for negative numbers:

julia> digits(ZZ(-3))
1-element Vector{Int64}:
 7

julia> digits(-3)
1-element Vector{Int64}:
 -3

julia> digits(BigInt(-3))
1-element Vector{Int64}:
 -3

From the tests, I would assume that the result for ZZRingElem and BigInt should coincide; the tests are of course only for positive numbers though.
The issue is that Nemo.divrem is not the same as Base.divrem as I learnt from this comment here:

Nemo.jl/src/flint/fmpz.jl

Line 629 in 659462d

# N.B. Base.divrem differs from Nemo.divrem

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (659462d) 84.23% compared to head (a022a57) 84.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1590      +/-   ##
==========================================
- Coverage   84.23%   84.21%   -0.02%     
==========================================
  Files          94       94              
  Lines       36671    36671              
==========================================
- Hits        30888    30882       -6     
- Misses       5783     5789       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.