refactor: change system parameters from traits to structs

[SOF3]

also fixes #51

[SOF3]

also fixes #51 since the Accessor trait has been rewritten and the Storage traits are no longer unsafe.

[SOF3]

Some broken doc links still need fixing

[codecov-commenter]

Codecov Report

Attention: 169 lines in your changes are missing coverage. Please review.

Comparison is base (b3131e4) 65.51% compared to head (c4c08f2) 67.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   65.51%   67.70%   +2.18%     
==========================================
  Files          75       86      +11     
  Lines        5133     6314    +1181     
==========================================
+ Hits         3363     4275     +912     
- Misses       1770     2039     +269     

Files	Coverage Δ
src/comp/any.rs	97.14% <ø> (ø)
src/comp/discrim.rs	95.50% <ø> (ø)
src/entity.rs	100.00% <100.00%> (ø)
src/entity/ealloc/snapshot.rs	77.27% <100.00%> (-22.73%)	⬇️
src/macros.rs	71.42% <100.00%> (-3.58%)	⬇️
src/storage.rs	100.00% <100.00%> (ø)
src/storage/tests.rs	100.00% <100.00%> (ø)
src/storage/tree.rs	100.00% <100.00%> (ø)
src/system.rs	0.00% <ø> (ø)
src/system/access/isotope.rs	100.00% <100.00%> (ø)
... and 39 more

... and 2 files with indirect coverage changes

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[SOF3]

Still missing par_entities_with and par_chunks_with