BandedBlockBandedMatrix function barrier

[jishnub]

This improves performance slightly
On master

julia> D = Derivative(Chebyshev()) ⊗ Derivative(Chebyshev());

julia> D2 = D[Fun((x,y) -> x^2*y^2, Chebyshev()^2)];

julia> DBv = view(D2, Block(1):Block(20), Block(1):Block(20));

julia> @btime BandedBlockBandedMatrix(Zeros, $DBv);
  6.338 μs (16 allocations: 104.77 KiB)

This PR

julia> @btime BandedBlockBandedMatrix(Zeros, $DBv);
  5.513 μs (11 allocations: 104.23 KiB)

[codecov]

Codecov Report

Base: 71.39% // Head: 71.56% // Increases project coverage by +0.17% 🎉

Coverage data is based on head (3f30013) compared to base (fea3939).
Patch coverage: 97.43% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
+ Coverage   71.39%   71.56%   +0.17%     
==========================================
  Files          79       79              
  Lines        8323     8314       -9     
==========================================
+ Hits         5942     5950       +8     
+ Misses       2381     2364      -17     

Impacted Files	Coverage Δ
src/LinearAlgebra/helper.jl	66.04% <90.90%> (+1.34%)	⬆️
src/Multivariate/TensorSpace.jl	86.26% <100.00%> (-0.72%)	⬇️
src/Operators/SubOperator.jl	83.16% <100.00%> (+1.48%)	⬆️
src/Operators/general/InterlaceOperator.jl	86.36% <100.00%> (+0.34%)	⬆️
src/Operators/general/algebra.jl	92.75% <100.00%> (ø)
src/Operators/spacepromotion.jl	66.19% <100.00%> (+1.40%)	⬆️
src/Operators/general/OperatorFunction.jl	54.16% <0.00%> (-8.34%)	⬇️
src/Multivariate/VectorFun.jl	55.20% <0.00%> (-4.17%)	⬇️
src/ApproxFunBase.jl	79.16% <0.00%> (-4.17%)	⬇️
src/Operators/ldiv.jl	77.77% <0.00%> (-3.71%)	⬇️
... and 33 more

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