Implement Extend for World

[Anders429]

This fixes #14.

[codecov-commenter]

Codecov Report

Merging #96 (f0dbf12) into dev (cb43f5d) will decrease coverage by 0.34%.
The diff coverage is 89.76%.

@@            Coverage Diff             @@
##              dev      #96      +/-   ##
==========================================
- Coverage   95.15%   94.81%   -0.35%     
==========================================
  Files          57       61       +4     
  Lines        9428     9542     +114     
==========================================
+ Hits         8971     9047      +76     
- Misses        457      495      +38     

Impacted Files	Coverage Δ
src/query/view/seal.rs	100.00% <ø> (ø)
src/world/mod.rs	99.52% <57.14%> (-0.48%)	⬇️
src/entities/iter.rs	78.78% <78.78%> (ø)
src/entities/mod.rs	96.87% <100.00%> (-3.13%)	⬇️
src/entities/raw.rs	100.00% <100.00%> (ø)
src/entities/seal/into_raw.rs	100.00% <100.00%> (ø)
src/entities/seal/length.rs	90.62% <100.00%> (ø)
src/entity/seal/unzip.rs	100.00% <100.00%> (ø)
src/entities/seal/storage.rs	38.46% <0.00%> (-36.93%)	⬇️

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[Anders429]

This is currently in development. While this solution works, it has some significant performance penalties for inserting into a World's Archetype that doesn't have any values already in it. It would be great if there were some way the specialized FromIterator<vec::IntoIter> implementation for Vec could be utilized, but that might not actually be possible.

I'm hesitant to merge this if it hurts performance enough. I'm currently seeing about a 3x increase in performance, which is significant (although it does put the performance right there next to the other ECS libraries, instead of way out in front of them like it currently is). While extending from an iterator is something that would be nice to have, I'm not sure that it is strictly necessary for the large amount of use cases. Most people will be just inserting entities directly.

[Anders429]

Note: here was the proof of concept for the Unzip heterogeneous list trait I added: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c9497548e94e343ac177ac3148ac991f

[Anders429]

As it currently stands, this PR is not going to be merged. It has grown stale over the last several months, and it is not the correct solution to the problem.

This feature may not actually be possible until specialization is stabilized.