Define dataids for BlockArray

[tkf]

Previously, Base.dataids(::BlockArray) was calling objectid which was rather slow:

julia> x = BlockArray(randn(6), 1:3);

julia> @btime Base.objectid($x)  # equivalent to dataids($x) before this PR
  150.738 ns (0 allocations: 0 bytes)
0x34d4c35d64a7d6ef

julia> @btime Base.dataids($x)
  1.647 ns (0 allocations: 0 bytes)
(0x00007fc3cc79fc40, 0x00007fc3cc79fbd0)

This patch also makes alias detection slightly stronger for block arrays. But I then noticed that block arrays can't have proper dataids because the number of "data arrays" can't be determined by the type parameters (if we try to make it precise, dataids cannot be inferred and also it's not O(1) function anymore).

FYI we are trying to improve alias detection in Base. The discussion in Julia repo is not super relevant to this issue. But I wonder this should be taken into account if we were to redesign it in Base: JuliaLang/julia#29546

[codecov-io]

Codecov Report

Merging #65 into master will decrease coverage by 0.31%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   63.69%   63.37%   -0.32%     
==========================================
  Files          11       11              
  Lines         482      486       +4     
==========================================
+ Hits          307      308       +1     
- Misses        175      178       +3

Impacted Files	Coverage Δ
src/blocksizes.jl	63.88% <0%> (-2.78%)	⬇️
src/blockarray.jl	78.88% <100%> (+0.23%)	⬆️

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