Enable beman-tidy (--require-all mode) into beman.span

[neatudarius]

Issue: bemanproject/beman-tidy#334

[coveralls]

coverage: 100.0%. remained the same — neatudarius-patch-1 into main

[ssam18]

Hi @neatudarius, I pushed a small README fix here, CI should go green now.

Turning on --require-all promotes two recommendations into requirements, and both were failing:

• readme.implements the check wants the exact Implements: spelling (colon outside the bold) and a wg21.link/Pxxxx reference. We had Implements: and only linked cppreference / eel.is.
• release.godbolt_trunk_version we had no Compiler Explorer badge in the README at all.

With those two fixed, beman-tidy goes from 94.29% to 100% locally.

One thing I would like your call on: I put P2821R5 (span::at()) on the Implements line, since it's the only paper actually cited anywhere in the code (span.hpp). But the library clearly aims at more than that #2 (span over initializer_list) and #4 (tuple protocol for fixed-size spans) are pointing at other proposals. The check happily accepts a comma-separated list of papers, so if you'd rather have several listed there, tell me which and I'll update the line.

Also FYI: the badge points at a Compiler Explorer link I created https://godbolt.org/z/4vsvzbf7z. It builds the README usage example against beman_span trunk on CE and runs clean. Easy to swap for a different snippet if you'd prefer something else there.