strip tuple wrapper to speed up centroid calc

[tlnagy]

continuation of JuliaImages/Images.jl#810

JuliaLang/julia#28980 seems unrelated because the 100x fold regression is fixed on master but this PR still yields about a 10% increase in median speed:

Images master

julia> a = rand(0:250, 1024, 1024);

julia> @benchmark Images.component_centroids(a)
BenchmarkTools.Trial: 
  memory estimate:  10.17 KiB
  allocs estimate:  4
  --------------
  minimum time:     1.972 ms (0.00% GC)
  median time:      2.748 ms (0.00% GC)
  mean time:        2.742 ms (0.00% GC)
  maximum time:     5.276 ms (0.00% GC)
  --------------
  samples:          1819
  evals/sample:     1

This PR

julia> @benchmark Images.component_centroids(a)
BenchmarkTools.Trial: 
  memory estimate:  10.31 KiB
  allocs estimate:  5
  --------------
  minimum time:     1.907 ms (0.00% GC)
  median time:      2.498 ms (0.00% GC)
  mean time:        2.489 ms (0.00% GC)
  maximum time:     3.595 ms (0.00% GC)
  --------------
  samples:          2000
  evals/sample:     1

julia> println(VERSION)
1.3.0-DEV.320

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   92.88%   92.94%   +0.05%     
==========================================
  Files           6        6              
  Lines         253      255       +2     
==========================================
+ Hits          235      237       +2     
  Misses         18       18

Impacted Files	Coverage Δ
src/connected.jl	88.72% <100%> (+0.17%)	⬆️

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[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   92.88%   92.94%   +0.05%     
==========================================
  Files           6        6              
  Lines         253      255       +2     
==========================================
+ Hits          235      237       +2     
  Misses         18       18

Impacted Files	Coverage Δ
src/connected.jl	88.72% <100%> (+0.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ecd67e2...6bf89b6. Read the comment docs.

[timholy]

Thanks!