Add divexact!, lcm!, submul!

[fingolfin]

Also refactor the code defining them to use macros to reduce
repetition substantially.

Finally merge and update the sections discussion unsafe operators
in the "rings" and "ring interface" chapters of the manual: the latter
now

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.04%. Comparing base (efb6ceb) to head (9732c44).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/fundamental_interface.jl	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1812      +/-   ##
==========================================
+ Coverage   88.03%   88.04%   +0.01%     
==========================================
  Files         119      119              
  Lines       30057    30045      -12     
==========================================
- Hits        26460    26454       -6     
+ Misses       3597     3591       -6     

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[lgoettgens]

apart from the comments, fine with me. I am not a big fan of @eval for these small loops, but we can of course unroll them again later, if we want to change only one of them

[fingolfin]

Comments addressed.

I don't love @eval for this either (it makes it harder to grep for declarations) I went for it here because I want to have the argument names consistent (z,a,b) and also the texts and there was a ton of repetition which is also error prone. In the end this saved me a lot of time and is more likely to be correct. If someone wants to unroll it again down the road, I don't mind, as long as I don't have to update the duplicated docstrings and implementations again later on :-).

The current setup also makes it super easy to add more of these... e.g. remove! and valuation! might be candidates. (Also set!(a,b) but we don't have a eval loop for that -- yet)

[lgoettgens]

Can you add HeckeCI, NemoCI and SingularCI to the "required" list? I am afraid to use auto-merge otherwise as it could just ignore those CI results.