Make MatrixNormal sampling non-allocating

[projekter]

Currently, sampling from MatrixNormal always allocates multiple temporaries: one of the matrix X that contains the normally-sampled values, one for the matrix product A * X, and one for the matrix product (A * X) * B. However, given that A and B are triangular (and square) matrices, all products can be done in-place and the sample can directly be stored to the output. So the method doesn't need to allocate at all, provided the Cholesky factors are available without allocation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.36%. Comparing base (0421b18) to head (e011669).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2012   +/-   ##
=======================================
  Coverage   86.36%   86.36%           
=======================================
  Files         146      146           
  Lines        8786     8788    +2     
=======================================
+ Hits         7588     7590    +2     
  Misses       1198     1198           

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[devmotion]

Looks good to me. Could you add a test for it, to ensure that it doesn't regress silently?