Add ldg function to Range

[sethrj]

This adds ldg friends to Range andn ItemMap to support loading from const collections. It also replaces troublesome implicit constructors with standard explicit ones.

[github-actions]

Test summary

 5 701 files   9 289 suites   18m 41s ⏱️
 2 326 tests  2 283 ✅  43 💤 0 ❌
32 901 runs  32 730 ✅ 171 💤 0 ❌

Results for commit 4fa7592.

♻️ This comment has been updated with latest results.

[pcanal]

LGTM.

[pcanal]

However the Windows failure is odd ... :(

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.31%. Comparing base (d10bc05) to head (4fa7592).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2426      +/-   ##
===========================================
- Coverage    87.32%   87.31%   -0.01%     
===========================================
  Files         1401     1401              
  Lines        44444    44446       +2     
  Branches     13495    13958     +463     
===========================================
+ Hits         38809    38810       +1     
- Misses        4410     4564     +154     
+ Partials      1225     1072     -153     

Files with missing lines	Coverage Δ
src/corecel/OpaqueId.hh	100.00% <ø> (ø)
src/corecel/cont/Range.hh	100.00% <100.00%> (ø)
src/corecel/cont/detail/LdgSpanImpl.hh	100.00% <100.00%> (ø)
src/corecel/cont/detail/RangeImpl.hh	99.00% <100.00%> (+0.01%)	⬆️
src/corecel/data/Collection.hh	100.00% <ø> (ø)
src/corecel/data/Ldg.hh	90.00% <ø> (ø)

... and 113 files with indirect coverage changes

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[sethrj]

However the Windows failure is odd ... :(

It's not clear whether this was an MSVC compiler bug (likely) or clang and GCC being permissive, but somehow it seems that the namespace lookup was being broken by a combination of templates, friends, and detail namespaces: adding using ::celeritas::ldg allows it to resolve.

I'm glad we have MSVC on our CI, because it would've been a hell of a lot more complicated to track down with 50k other changes…