Improve inference in blockintercer iteration

[jishnub]

On master

julia> s = Fourier();

julia> b = ApproxFunBase.BlockInterlacer(s);

julia> @btime first($b, 10);
  6.564 μs (124 allocations: 6.95 KiB)

This PR

julia> @btime first($b, 10);
  1.103 μs (28 allocations: 2.31 KiB)

[codecov]

Codecov Report

Base: 71.97% // Head: 71.97% // No change to project coverage 👍

Coverage data is based on head (41f28aa) compared to base (6b777fe).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #362   +/-   ##
=======================================
  Coverage   71.97%   71.97%           
=======================================
  Files          79       79           
  Lines        8311     8311           
=======================================
  Hits         5982     5982           
  Misses       2329     2329           

Impacted Files	Coverage Δ
src/LinearAlgebra/helper.jl	64.23% <100.00%> (ø)

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