Limit factorization size method to Circulant

[jishnub]

With this method, a Circulant satisfies

julia> C = Circulant(1:4);

julia> size(C) == size(factorize(C))
true

This doesn't hold for other AbstractToeplitz types given the current definition, so it's best to limit these for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (135d562) 95.63% compared to head (fd24ac4) 95.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #130   +/-   ##
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  Coverage   95.63%   95.63%           
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  Files           9        9           
  Lines         963      963           
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  Hits          921      921           
  Misses         42       42           

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