50 Allow systems to access components from multiple entities at once

[EdvinNilsson]

This pull request adds the Query system parameter that allows a system to access components from entities other than the currently iterated. Query implements IntoIterator and it can be used to iterate over queried entities. Here is an example how Query can be used:

fn system(query: Query<(Read<A>, Read<B>)>) {
    for (a, b) in query {

    }
}

Closes #50

[github-actions]

Benchmark for 80d5950

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Test	Base	PR	%
sleep	166.9±12.74µs	162.8±2.78µs	-2.46%
sleep x/1	61.9±2.98µs 15.8 KElem/sec	N/A	N/A
sleep x/2	66.7±4.93µs 29.3 KElem/sec	N/A	N/A
sleep x/3	65.7±2.57µs 44.6 KElem/sec	N/A	N/A
sleep x/4	65.5±2.33µs 59.6 KElem/sec	N/A	N/A
sleep x/5	70.7±14.07µs 69.1 KElem/sec	N/A	N/A
sleep x/6	66.9±2.84µs 87.5 KElem/sec	N/A	N/A
sleep x/7	69.5±3.71µs 98.4 KElem/sec	N/A	N/A
sleep x/8	72.6±7.49µs 107.6 KElem/sec	N/A	N/A
sleep x/9	71.2±2.02µs 123.4 KElem/sec	N/A	N/A

[martinjonsson01]

LGTM! Nicely done.

[github-actions]

Benchmark for 4e5460e

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Test	Base	PR	%
sleep	167.3±7.91µs	170.1±11.48µs	+1.67%
sleep x/1	67.1±7.54µs 14.6 KElem/sec	N/A	N/A
sleep x/2	66.9±5.89µs 29.2 KElem/sec	N/A	N/A
sleep x/3	71.0±6.70µs 41.3 KElem/sec	N/A	N/A
sleep x/4	72.7±12.38µs 53.7 KElem/sec	N/A	N/A
sleep x/5	71.7±7.86µs 68.1 KElem/sec	N/A	N/A
sleep x/6	71.6±5.02µs 81.9 KElem/sec	N/A	N/A
sleep x/7	73.0±9.73µs 93.7 KElem/sec	N/A	N/A
sleep x/8	75.4±10.33µs 103.6 KElem/sec	N/A	N/A
sleep x/9	75.8±12.30µs 115.9 KElem/sec	N/A	N/A

[ChristofferPerssons]

Looks good!

Quite interested in how performance of a system is affected when intra-system parallelization is disallowed and whether or not it would be more performant and possible to bypass this by uses of commands that add component flags to specific entities instead.

[github-actions]

Benchmark for 148ffb6

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Test	Base	PR	%
sleep	158.8±2.02µs	158.8±1.19µs	0.00%
sleep x/1	59.6±0.89µs 16.4 KElem/sec	N/A	N/A
sleep x/2	60.6±0.88µs 32.2 KElem/sec	N/A	N/A
sleep x/3	61.7±0.88µs 47.5 KElem/sec	N/A	N/A
sleep x/4	62.7±1.39µs 62.3 KElem/sec	N/A	N/A
sleep x/5	63.7±1.38µs 76.7 KElem/sec	N/A	N/A
sleep x/6	64.9±0.79µs 90.3 KElem/sec	N/A	N/A
sleep x/7	65.9±1.41µs 103.8 KElem/sec	N/A	N/A
sleep x/8	66.8±1.15µs 117.0 KElem/sec	N/A	N/A
sleep x/9	67.6±0.81µs 129.9 KElem/sec	N/A	N/A

[github-actions]

Benchmark for 734b3d2

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Test	Base	PR	%
sleep	162.9±4.58µs	166.6±7.48µs	+2.27%
sleep x/1	62.6±2.53µs 15.6 KElem/sec	N/A	N/A
sleep x/2	65.5±4.50µs 29.8 KElem/sec	N/A	N/A
sleep x/3	65.2±3.79µs 44.9 KElem/sec	N/A	N/A
sleep x/4	66.7±2.32µs 58.5 KElem/sec	N/A	N/A
sleep x/5	68.2±3.55µs 71.6 KElem/sec	N/A	N/A
sleep x/6	71.4±4.35µs 82.0 KElem/sec	N/A	N/A
sleep x/7	70.7±6.70µs 96.7 KElem/sec	N/A	N/A
sleep x/8	70.2±3.30µs 111.2 KElem/sec	N/A	N/A
sleep x/9	72.3±4.19µs 121.6 KElem/sec	N/A	N/A

[github-actions]

Benchmark for bdc7d03

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Test	Base	PR	%
sleep	159.0±0.99µs	159.4±2.65µs	+0.25%
sleep x/1	59.5±0.56µs 16.4 KElem/sec	N/A	N/A
sleep x/2	61.9±3.13µs 31.5 KElem/sec	N/A	N/A
sleep x/3	61.8±1.38µs 47.4 KElem/sec	N/A	N/A
sleep x/4	62.8±1.94µs 62.2 KElem/sec	N/A	N/A
sleep x/5	65.2±3.74µs 74.9 KElem/sec	N/A	N/A
sleep x/6	64.8±1.26µs 90.5 KElem/sec	N/A	N/A
sleep x/7	66.0±1.95µs 103.6 KElem/sec	N/A	N/A
sleep x/8	67.0±2.15µs 116.6 KElem/sec	N/A	N/A
sleep x/9	69.2±4.43µs 127.0 KElem/sec	N/A	N/A

[github-actions]

Benchmark for 44f8852

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Test	Base	PR	%
sleep	161.5±3.87µs	158.8±0.77µs	-1.67%
sleep x/1	59.6±1.04µs 16.4 KElem/sec	N/A	N/A
sleep x/2	61.4±2.98µs 31.8 KElem/sec	N/A	N/A
sleep x/3	63.1±3.38µs 46.4 KElem/sec	N/A	N/A
sleep x/4	62.6±1.77µs 62.4 KElem/sec	N/A	N/A
sleep x/5	63.5±1.30µs 76.9 KElem/sec	N/A	N/A
sleep x/6	64.4±1.55µs 90.9 KElem/sec	N/A	N/A
sleep x/7	66.5±1.96µs 102.9 KElem/sec	N/A	N/A
sleep x/8	68.1±2.91µs 114.7 KElem/sec	N/A	N/A
sleep x/9	67.5±1.09µs 130.3 KElem/sec	N/A	N/A