Special case identity permutations and improve printing of permutations

[ChrisJefferson]

Resolves #3547

This adds special cases for identity permutations in various methods. These do get called fairly often, and are very cheap.

EDIT: This now fixes another problem found along the way. GAP has the internal concept of the 'degree' of a permutation, which can be larger than the LargestMovedPoint. For example, p=(1,2) has DEG_PERM=2, but q=(2^19,2^20)*(1,2)*(2^19,2^20) has DEG_PERM=2^20, as GAP doesn't try hard to reduce this value, but just uses a weak guess.

This value is used in printing to decide how many digits we need when printing, so the permutations p and q print differently, even though from the GAP language there is no way to tell them apart. This could break tests in some packages, who have made permutations with a large DEG_PERM

 gap> p := (1,2);
(1,2)
gap> q := (1,2)*(2^19,2^20)*(2^19,2^20);
(1,2)
gap> Print(p);
(1,2)
gap> Print(q);
(    1,    2)
gap> p=q;
true

Text for release notes

• Operations involving identity permutations optimised.
• Improve printing of permutations

[codecov]

Codecov Report

Merging #3566 into master will decrease coverage by 8.89%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3566      +/-   ##
==========================================
- Coverage   84.53%   75.64%    -8.9%     
==========================================
  Files         696      644      -52     
  Lines      345073   296213   -48860     
==========================================
- Hits       291716   224067   -67649     
- Misses      53357    72146   +18789

Impacted Files	Coverage Δ
src/permutat.cc	85.38% <100%> (-11.75%)	⬇️
src/fibhash.h	0% <0%> (-100%)	⬇️
src/syntaxtree.c	5.44% <0%> (-90.72%)	⬇️
lib/meatauto.gi	5.99% <0%> (-89.32%)	⬇️
lib/nilpquot.gi	11.11% <0%> (-88.89%)	⬇️
src/modules.h	16.66% <0%> (-83.34%)	⬇️
lib/helpt2t.gi	0.23% <0%> (-83.14%)	⬇️
src/dteval.c	3.3% <0%> (-80.12%)	⬇️
src/objset.c	8.13% <0%> (-76.74%)	⬇️
lib/dt.g	7.75% <0%> (-72.2%)	⬇️
... and 419 more

[coveralls]

Coverage increased (+0.001%) to 84.316% when pulling 1461dba on ChrisJefferson:perm-id into d9b9d3a on gap-system:master.

[fingolfin]

Is there a strong reason to combine a potentially breaking change with an enhancement in one PR? I think we might want to merge one immediately, but for the other take some more time to first let Alex run all package tests and determine if we break some packages, and then give the package authors a chance to adapt (or even help them fix their packages)?

[fingolfin]

The enhancement seems obvious enough (how useful it is in practice is another question, but I'd expect little harm). I am reluctant about merging it mixed in with a potentially breaking change to printing, though (even though the "break" should "only" affect package tests).

[ChrisJefferson]

I'll have a rearrange. The two issues ended up merged because I only discovered the problem with printing when testing this.

[ChrisJefferson]

So, I looked at separating this, but I think it might be best to test them together, as they can both produce visible differences in tests, so if we separate we just have to make two passes over the packages.

[fingolfin]

Sadly tests seem to be failing?

[fingolfin]

Great! I have one tiny further optimization suggestion, but it's also OK to merge this directly, you are probably fed enough by my niggling :-). Thank you for making this perfect

[fingolfin]

Still great. And as to potentially breaking package tests, I guess we can argue that they were not very robust to start with ;-). After merging this, let's try to watch the Travis package tests at https://travis-ci.org/gap-infra/gap-docker-pkg-tests-master in the coming days for new failures