Use ArrowProxy for iterators to remove first()/second() api discrepancy

[alexkaratarakis]

ArrowProxy vs existing (PairView-based) implementation:

Pros:

• No API discrepancy

Cons:

• Does not return an l-value reference, but a value. This means: 1) Need to suppress -Wrange-loop-bind-reference globally because they trigger on common loops (in contrast to std::map):

  for (const auto& key_and_value : s) { }
  
  for (const auto& [key, value] : s) { }

  Relevant: https://quuxplusone.github.io/blog/2020/08/26/wrange-loop-analysis/ https://quuxplusone.github.io/blog/2018/12/15/autorefref-always-works/ flat_map work the same way: https://godbolt.org/z/r187d7zn7

  2) These non-cost & don't work at all (in contrast to std::map):

      // error: non-const lvalue reference to type 'pair<...>' cannot bind to a temporary of type
      // 'pair<...>'
     for (auto& key_and_value : s) { }
  
      // error: non-const lvalue reference to type 'pair<...>' cannot bind to a temporary of type
      // 'pair<...>'
      for (auto& [key, value] : s) { }

  3) range-v3 has issues (but not std::ranges, std::map works with either):

  int first_entry = (*f.begin()).second;  // Can't use arrow with range-v3 because it requires
                                          // l-value. Note that std::ranges works

All of the above is demo'ed in the unit tests.

Other option that returns l-value was explored: https://github.com/teslamotors/fixed-containers/pull/38 Comparison of the l-value approach with ArrowProxy https://github.com/teslamotors/fixed-containers/pull/39

[alexkaratarakis]

Closing in favor of: https://github.com/teslamotors/fixed-containers/pull/45 https://github.com/teslamotors/fixed-containers/pull/48