extend AbstractMatrix instead of similar

[jishnub]

This makes the following work:

julia> C = Circulant([1,2,0,0])
4×4 Circulant{Int64, Vector{Int64}}:
 1  0  0  2
 2  1  0  0
 0  2  1  0
 0  0  2  1

julia> D = Diagonal(axes(C,2))
4×4 Diagonal{Int64, Base.OneTo{Int64}}:
 1  ⋅  ⋅  ⋅
 ⋅  2  ⋅  ⋅
 ⋅  ⋅  3  ⋅
 ⋅  ⋅  ⋅  4

julia> C * D
4×4 Matrix{Int64}:
 1  0  0  8
 2  2  0  0
 0  4  3  0
 0  0  6  4

julia> C = Circulant([1,2,0,2])
4×4 Circulant{Int64, Vector{Int64}}:
 1  2  0  2
 2  1  2  0
 0  2  1  2
 2  0  2  1

julia> sort(eigvals(C), by=reim) ≈ eigvals(Symmetric(C))
true

similar should return a mutable collection that has setindex! defined.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.23 🎉

Comparison is base (7558d4a) 94.72% compared to head (245c557) 94.96%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   94.72%   94.96%   +0.23%     
==========================================
  Files           7        7              
  Lines         796      794       -2     
==========================================
  Hits          754      754              
+ Misses         42       40       -2     

Impacted Files	Coverage Δ
src/ToeplitzMatrices.jl	100.00% <ø> (ø)
src/hankel.jl	96.15% <100.00%> (+2.48%)	⬆️
src/special.jl	98.55% <100.00%> (ø)
src/toeplitz.jl	97.53% <100.00%> (-0.04%)	⬇️

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[putianyi889]

can you add tests for #84 ?

[coveralls]

Pull Request Test Coverage Report for Build 5134623567

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 5 of 5 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 94.987%

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Totals
Change from base Build 5118652295:	0.2%
Covered Lines:	739
Relevant Lines:	778

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💛 - Coveralls