eigen for tridiagonal Fill

[jishnub]

This moves the Toeplitz functions from JuliaArrays/FillArrays.jl#256, so that eigen/eigvals for tridiagonal matrices with Fill bands become O(n)/O(n^2) after loading this package. This is type-piracy, but it was suggested that this be added to this package.

julia> using FillArrays, LinearAlgebra, ToeplitzMatrices, BenchmarkTools

julia> S = SymTridiagonal(Fill(4.0, 5000), Fill(3.0, 4999));

julia> @btime eigvals($S);
  153.141 μs (2 allocations: 39.11 KiB)

[codecov]

Codecov Report

Patch coverage: 95.50% and project coverage change: +0.03 🎉

Comparison is base (49bde1d) 94.98% compared to head (0c805db) 95.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   94.98%   95.02%   +0.03%     
==========================================
  Files           7        8       +1     
  Lines         798      884      +86     
==========================================
+ Hits          758      840      +82     
- Misses         40       44       +4     

Impacted Files	Coverage Δ
src/ToeplitzMatrices.jl	87.50% <81.81%> (-4.81%)	⬇️
src/eigen.jl	96.92% <96.92%> (ø)
src/hankel.jl	96.20% <100.00%> (+0.04%)	⬆️
src/special.jl	99.33% <100.00%> (+0.03%)	⬆️
src/toeplitz.jl	97.56% <100.00%> (+0.03%)	⬆️

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