perf: add 0-copy reduce-vcat specialization for VectorOfVectors

[pfackeldey]

This PR adds a specialization for reduce(vcat, VoV) specialization to directly return a flatview, this operation is zero-copy.

[oschulz]

Let's test the behavior

@test reduce(vcat, V1) == reduce(vcat, Array(V1))

instead.

[oschulz]

See other comments.

[Moelf]

See other comments.

is there some forgotten comments due to Github UI?

[oschulz]

is there some forgotten comments due to Github UI?

No, I meant the change comment, I just had clicked "approve" by default, so I needed to add another review with "request changes".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.21%. Comparing base (021083f) to head (b82e295).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   96.20%   96.21%           
=======================================
  Files           8        8           
  Lines         527      528    +1     
=======================================
+ Hits          507      508    +1     
  Misses         20       20           

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[oschulz]

@pfackeldey are you going to add reduce for ArrayOfSimilarArrays to this PR? Then I'd hold off with the merge.

[pfackeldey]

@pfackeldey are you going to add reduce for ArrayOfSimilarArrays to this PR? Then I'd hold off with the merge.

Ah, I didn't know this reduce + vcat specialization can also be done here for ArrayOfSimilarArrays. Then, yes, I'll add this soon to this PR 👍

[oschulz]

You can shorten this as

Suggested change

	V1 = VectorOfVectors(ref_AoA1(Float32, 1))
	V2 = VectorOfVectors(ref_AoA1(Float32, 2))
	V3 = VectorOfVectors(ref_AoA1(Float32, 3))
	V4 = VectorOfVectors(ref_AoA1(Float32, 4))
	@test reduce(vcat, V1) == reduce(vcat, Array(V1))
	@test reduce(vcat, V2) == reduce(vcat, Array(V2))
	@test reduce(vcat, V3) == reduce(vcat, Array(V3))
	@test reduce(vcat, V4) == reduce(vcat, Array(V4))
	@test (@allocated reduce(vcat, V1)) == 0
	@test (@allocated reduce(vcat, V2)) == 0
	@test (@allocated reduce(vcat, V3)) == 0
	@test (@allocated reduce(vcat, V4)) == 0
	for n in [1, 2, 3, 4]
	V = VectorOfVectors(ref_AoA1(Float32, n))
	@test reduce(vcat, V) == reduce(vcat, Array(1))
	@test (@allocated reduce(vcat, V)) == 0
	end