rwlock (and bswap8 because oops)

[lethalbit]

This is just an initial draft of rwlock for #18, as it uses std::shared_mutex it's exclusive to C++17 or newer.

[codecov]

Codecov Report

Merging #38 (49a0195) into main (37e0e2f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   96.25%   96.26%   +0.01%     
==========================================
  Files          52       52              
  Lines        3735     3745      +10     
  Branches      276      276              
==========================================
+ Hits         3595     3605      +10     
  Misses        128      128              
  Partials       12       12              

Impacted Files	Coverage Δ
substrate/bits	100.00% <100.00%> (ø)
test/bits.cxx	100.00% <100.00%> (ø)

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[dragonmux]

It's looking pretty good - the only changes we've noted (and we've only noted the first instance of each to keep the review light) are really stylistic consistency with the rest of the code base, and a note about how SUBSTRATE_NO_DISCARD() was intended to be used

We don't mind if you want to fix the points before merge or have us fix them after in a clean-up pass - just let us know which you'd like to do and we'll adjust the approval accordingly. Exciting to soon have a proper rwlock type available

[dragonmux]

LGTM, thanks for fixing those items - we'll merge this just as soon as the builds complete