Fix QuotientMod docs, and the integer implementation

[fingolfin]

This partially reverts changes we made 2013. The documentation
is now correct (resp. consistent again), and several corner cases
work correctly. E.g. QuotientMod(0,0,m) never worked correctly, even
though by the definition given in the manual, 1 is a valid result.

Fixes #149

[codecov]

Codecov Report

Merging #1991 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1991      +/-   ##
==========================================
+ Coverage   65.94%   65.95%   +<.01%     
==========================================
  Files         898      898              
  Lines      273206   273679     +473     
  Branches    12771    11949     -822     
==========================================
+ Hits       180175   180513     +338     
- Misses      90211    90312     +101     
- Partials     2820     2854      +34

Impacted Files	Coverage Δ
lib/integer.gi	78.1% <100%> (+0.58%)	⬆️
hpcgap/lib/integer.gi	63.88% <100%> (+0.88%)	⬆️
lib/queue.g	66.4% <0%> (-3.2%)	⬇️
src/hpc/thread.c	45.41% <0%> (-1%)	⬇️
src/funcs.c	76.24% <0%> (-0.27%)	⬇️
src/objset.c	82.59% <0%> (-0.24%)	⬇️
src/hpc/threadapi.c	34.64% <0%> (-0.1%)	⬇️
src/stats.c	79.3% <0%> (ø)	⬆️
src/streams.c	47.58% <0%> (+0.42%)	⬆️
hpcgap/lib/zmodnz.gi	94.92% <0%> (+1.08%)	⬆️
... and 7 more

[fingolfin]

@alex-konovalov this is reverting resp. adjusting some changes you made back then, so perhaps you have some thoughts on this?

[olexandr-konovalov]

For example, what's changed:

1. In GAP 4.8.8, both QuotientMod(4, 2, 6); and QuotientMod(2, 4, 6); return fail. With this PR, the former is 2, the latter is fail.

2. for a := ZmodnZObj(2, 6); and b := ZmodnZObj(4, 6);, both b/a and a/b return fail in GAP 4.8.8. With this PR, we have

gap> b/a;
ZmodnZObj( 2, 6 )
gap> a/b;
fail

Shouldn't both cases in the 2nd example return a non-fail result? In the first example we have a Euclidean ring, and its new output now matches the documentation, so fine. But what about the 2nd example?

Otherwise, I ran GAP test suite in Jenkins for this PR, and did not discover any regressions.

[fingolfin]

@alex-konovalov you are 100% right, I am glad you paid attention and double checked! In fact, also the result for QuotientMod(0,0,m) was wrong -- it must be 0, per the documentation (which apparently I did not read carefully enough sigh).

All of this should be resolved now. Also, I changed the code to use INVMODINT instead of going through mod for rationals. I.e. a tiny performance improvement.

[olexandr-konovalov]

Thanks @fingolfin - now looks good, and I've rerun Jenkins test to see if it has any impact on package tests - apparently none.

[mohamed-barakat]

Excellent, thank you both very much.