Add SortedList method with function

[james-d-mitchell]

This PR adds a method for SortedList that takes a function as second argument.

Resolves #1002.

[coveralls]

Coverage increased (+0.01%) to 85.152% when pulling 22bb293 on james-d-mitchell:sortedlist into 49bec58 on gap-system:master.

[james-d-mitchell]

I’m pretty sure that if the appveyor tests are re run this will pass, the failure looks like it happened because I force pushed almost immediately after opening the PR

[fingolfin]

Thanks, this is really helpful; but I think the documentation should be a tiny bit clearer.

[fingolfin]

Minor (completely optional) suggestion: Even better would be to also test that the input is not modified. E.g. by passing an immutable list to the function.

[codecov]

Codecov Report

Merging #3376 into master will decrease coverage by <.01%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #3376      +/-   ##
==========================================
- Coverage   85.14%   85.13%   -0.01%     
==========================================
  Files         697      697              
  Lines      344069   344080      +11     
==========================================
+ Hits       292941   292946       +5     
- Misses      51128    51134       +6

Impacted Files	Coverage Δ
lib/coll.gd	95.09% <100%> (+0.01%)	⬆️
lib/coll.gi	96.85% <88.88%> (-0.04%)	⬇️
src/sysmem.c	56.39% <0%> (-0.59%)	⬇️
src/gap.c	79.74% <0%> (-0.32%)	⬇️
src/stats.c	94.93% <0%> (-0.21%)	⬇️
src/streams.c	73.61% <0%> (-0.12%)	⬇️

[fingolfin]

(and feel free to squash any suggested changes by me, I do not need "credit" for them in any form either ;-). And don't worry about AppVeyor failing, that is simply broken right now (but hopefully I can get it fixed via PR #3377)

[fingolfin]

I took the liberty of adding Resolves #1002. to the issue description, so that this issue gets closed when we merge this PR.

[DominikBernhardt]

This is my first review, so I might have missed something, but this looks good to me.