Simplify collection backend implementation

[sethrj]

This removes some of the extra templating and redundant traits classes used in collection. It should slightly reduce the amount of object code due to simplified templates, will make debugging easier due to one nested class implementation, and generally should be easier to read.

[github-actions]

Test summary

 5 955 files   9 652 suites   19m 26s ⏱️
 2 250 tests  2 208 ✅  42 💤 0 ❌
33 562 runs  33 435 ✅ 127 💤 0 ❌

Results for commit 3e029d0.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.26%. Comparing base (e2c8f84) to head (3e029d0).

Files with missing lines	Patch %	Lines
src/corecel/data/detail/CollectionImpl.hh	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2351   +/-   ##
========================================
  Coverage    87.26%   87.26%           
========================================
  Files         1377     1377           
  Lines        43836    43826   -10     
  Branches     13875    13866    -9     
========================================
- Hits         38253    38245    -8     
+ Misses        4524     4520    -4     
- Partials      1059     1061    +2     

Files with missing lines	Coverage Δ
src/celeritas/global/CoreTrackView.hh	87.50% <100.00%> (ø)
src/celeritas/track/SimData.hh	95.83% <ø> (ø)
src/corecel/data/Collection.hh	100.00% <100.00%> (ø)
src/corecel/data/DeviceAllocation.hh	76.47% <ø> (ø)
src/corecel/data/DeviceVector.hh	13.15% <ø> (ø)
src/corecel/data/LdgIterator.hh	97.01% <ø> (ø)
src/corecel/data/detail/CollectionImpl.hh	90.00% <87.50%> (+18.57%)	⬆️

... and 5 files with indirect coverage changes

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