!! C++20 like views

[rrahn]

Description !!!

Make all views as similar as possible to the C++20 standard, since this gives us the highest chance that they will work together in composable view definitions and are stable for the future. It is also a good practice, since the standard documents how they are implemented and every developer can follow these guidelines.

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Already discussed:

• C++20 "standard" conform #49
  Core meeting 2020-12-07: Everything is done already
• All views shall have explicit constructors https://github.com/seqan/seqan3/issues/941 still valid?
  Core meeting 2020-12-07: Yes, they should be explicit if only one argument is given, e.g. http://eel.is/c++draft/range.join which does the same.
  TODO: Make an issue for this. (Old issue is being re-used; I reopened)
• change behaviour of take/drop according to P1739/P1664
  Core meeting 2020-12-07:
  Declare take/drop view as experimental; The reason is that the standard take/drop view seems to have the same properties.
  TODO: This should be benchmarked and if there is a regression this should be backported to the standard.
• view::persist should downgrage ContiguousIterator/Range to RandomAccessIterator/Range because contiguous suggests you can directly access the memory whose lifetime might end, if not access via view::persist. Should we still offer view::persist or remove it entirely
  Core meeting 2020-12-07: We remove this as discussed already here https://github.com/orgs/seqan/projects/4#card-41487782
• Give up our own definitions in once some core language concepts without std::boolean broken in 0.9.x ericniebler/range-v3#1322 is resolved. Still valid?
  Core meeting 2020-12-07: This is done, see: Simplify seqan3/std/concepts #190
• some changes to views::pairwise_combine:
  • rename to views::all_pairs -> we decided against this
  • make view factory instead of adaptor
  • have single param interface (like now)
  • add two-param interface where all pairs between the sets are generated
  • change the tuple-type to be (seq1, seq2, id1, id2) where id1 and id2 are numbers that indicate the original IDs of the sequences

Acceptance Criteria

Tasks

Definition of Done

• Implementation and design approved
• Unit tests pass
• Test coverage = 100%
• Microbenchmarks added and/or affected microbenchmarks < 5% performance drop
• API documentation added
• Tutorial/teaching material added
• Test suite compiles in less than 30 seconds (on travis)
• Changelog entry added

[marehr]

2021-04-16 Core Meeting

views::pairwise_combine has a slight preferential over views::all_pairs, we'll keep the pairwise_combine name.

Wiki calls it k-combination, so in this case 2-combination which is closer to pairwise_combine.

[rrahn]

This came from @h-2. I just transferred it into the new issue system.

[marehr]

From my POV done; open a new issue if needed