Size check in 2-argument mul

[jishnub]

This PR adds a size check in the 2-argument mul, so that now the destination array is allocated only if the sizes of the arguments are compatible with matrix multiplication. This means that we don't allocate in case of an error anymore.

The performance for small-matrix multiplication seems largely similar (it's comparable to #1310, and seems identical within the noise limit):

julia> A = [1 2; 3 4];

julia> @btime $A * $A;
  42.304 ns (2 allocations: 112 bytes) # before this PR
  44.203 ns (2 allocations: 112 bytes) # this PR

We also redirect the generic mul to _mul now, which is the function that defines the multiplication code. This allows us to reuse the _mul definition elsewhere without having to repeat code. Currently, this is mainly necessary in the Bidiagonal-triangular multiplications.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.74%. Comparing base (e4e8c19) to head (f2c5004).
Report is 57 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1315      +/-   ##
==========================================
+ Coverage   93.72%   93.74%   +0.01%     
==========================================
  Files          34       34              
  Lines       15692    15694       +2     
==========================================
+ Hits        14708    14712       +4     
+ Misses        984      982       -2     

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