Reduce allocations in symmetric eigen

[jishnub]

ω = 25.0
d = -10..10;
S = Legendre(d);
V = Fun(x -> ω * x^2 + x^4, S)
L = -Derivative(S, 2) + V;
B = Dirichlet(S);
SE = ApproxFun.SymmetricEigensystem(L, B);

Using this,

julia> @btime ApproxFunBase.bandmatrices_eigen($SE, 100);
  3.108 ms (6871 allocations: 1.50 MiB) # master
  3.027 ms (6427 allocations: 1.48 MiB) # PR

[codecov]

Codecov Report

Patch coverage: 16.66% and no project coverage change.

Comparison is base (27c3236) 27.41% compared to head (1bd5c52) 27.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #532   +/-   ##
=======================================
  Coverage   27.41%   27.42%           
=======================================
  Files          80       80           
  Lines        8363     8366    +3     
=======================================
+ Hits         2293     2294    +1     
- Misses       6070     6072    +2     

Files Changed	Coverage Δ
src/eigen.jl	0.00% <0.00%> (ø)
src/Spaces/QuotientSpace.jl	3.95% <28.57%> (+0.50%)	⬆️

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