Define complextype, realtype, imagtype

[mtfishman]

Define type space versions of Base.complex, Base.real, and Base.imag, analogous to #51 which defines similartype as a type space version of Base.similar.

The motivation for this is making block sparse matrix SVD more generic, specifically this helps generalize these lines: https://github.com/ITensor/BlockSparseArrays.jl/blob/f9b971b7aafb2cdaf828160863da87676ba07f0f/src/factorizations/svd.jl#L27-L30 where we construct the block type of the matrix of singular values. (This is one approach to that, another approach could be to infer the block types of U, S, and V by calling Base.promote_op(svd_compact, blocktype(A)), I'll play around with both approaches but even if this PR isn't used for that particular case I think it is still useful.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.84%. Comparing base (8118760) to head (a433970).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
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+ Coverage   83.06%   83.84%   +0.78%     
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  Files          14       15       +1     
  Lines         248      260      +12     
==========================================
+ Hits          206      218      +12     
  Misses         42       42              

Flag	Coverage Δ
docs	37.74% <0.00%> (-1.85%)	⬇️

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