Support QuasiDiagonal

[dlfivefifty]

@jagot this would avoid the deprecation warning though I'm inclined to not merge this until the next breaking change tag

[codecov]

Codecov Report

Merging #61 (1b57646) into master (ce37dcd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 1b57646 differs from pull request most recent head 0bf4d2f. Consider uploading reports for the commit 0bf4d2f to get more accurate results

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   84.06%   84.07%   +0.01%     
==========================================
  Files          10       10              
  Lines        1406     1407       +1     
==========================================
+ Hits         1182     1183       +1     
  Misses        224      224              

Impacted Files	Coverage Δ
src/diagonal.jl	84.31% <100.00%> (ø)
src/memorylayout.jl	82.46% <100.00%> (+0.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce37dcd...0bf4d2f. Read the comment docs.

[jagot]

I don't need it soon, so merging this can wait.