Introduce QRPackedQMatrix

[dlfivefifty]

This is a version of QRPackedQ that conforms to the AbstractMatrix interface

[codecov]

Codecov Report

Attention: Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.

Project coverage is 92.12%. Comparing base (6874396) to head (3710276).

Files with missing lines	Patch %	Lines
src/qr.jl	43.75%	9 Missing ⚠️
src/MatrixFactorizations.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   92.64%   92.12%   -0.53%     
==========================================
  Files           9        9              
  Lines        1401     1409       +8     
==========================================
  Hits         1298     1298              
- Misses        103      111       +8     

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