Faster A[Block(1,2)] for BandedBlockBandedMatrix

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08% :tada:

Comparison is base (9476648) 86.98% compared to head (896f6ed) 87.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #166 +/- ## ========================================== + Coverage 86.98% 87.07% +0.08% ========================================== Files 11 11 Lines 1176 1184 +8 ========================================== + Hits 1023 1031 +8 Misses 153 153 ``` | [Files Changed](https://app.codecov.io/gh/JuliaLinearAlgebra/BlockBandedMatrices.jl/pull/166?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra) | Coverage Δ | | |---|---|---| | [src/BandedBlockBandedMatrix.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/BlockBandedMatrices.jl/pull/166?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL0JhbmRlZEJsb2NrQmFuZGVkTWF0cml4Lmps) | `87.25% <100.00%> (+0.40%)` | :arrow_up: |

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