Reduce allocations in _block_indices

[mcognetta]

_block_indices allocates two arrays that are the size of the number of blocks, no matter what the inputs are. This is quite slow, as it is done every time getindex, setindex, etc is called. It also makes a third allocation (calling sum on a slice of one of the aforementioned allocated arrays) but the size of this depends on the inputs.

This PR removes those allocations.

julia> x = [rand(5:30) for _ in 1:100]; y = Random.shuffle(x);

julia> z = BlockDiagonal([rand(x,y) for (x,y) in zip(x, y)]);

# before
julia> @allocated z[3, 3]
1888

# this PR
julia> @allocated z[3, 3]
96

[codecov]

Codecov Report

Merging #88 (db99cc6) into master (0b98861) will decrease coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   96.09%   95.87%   -0.23%     
==========================================
  Files           5        5              
  Lines         333      315      -18     
==========================================
- Hits          320      302      -18     
  Misses         13       13              

Impacted Files	Coverage Δ
src/blockdiagonal.jl	85.48% <100.00%> (-1.09%)	⬇️
src/linalg.jl	95.18% <0.00%> (-0.23%)	⬇️
src/base_maths.jl	100.00% <0.00%> (ø)

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[mzgubic]

Thanks for this! And apologies for the delay, it somehow slipped through my inbox. Added a suggestion, and if you could bump the version please then we are ready to merge.

[mcognetta]

Ah, sorry I didn't see the comments. Thanks for handling it!