Fix rewrite of +(x, *(y...)) into add_mul

[odow]

Closes #224

TODO

• Tests

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (e3f1da6) 89.51% compared to head (6cc2f52) 89.57%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   89.51%   89.57%   +0.05%     
==========================================
  Files          23       23              
  Lines        2137     2149      +12     
==========================================
+ Hits         1913     1925      +12     
  Misses        224      224              

Files Changed	Coverage Δ
src/rewrite_generic.jl	99.37% <100.00%> (+0.05%)	⬆️

... and 1 file with indirect coverage changes

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[odow]

So the JuMP master tests are failing because this hits some different methods.

In particular, we now hit:

MutableArithmetics.jl/src/MutableArithmetics.jl

Line 27 in 01cae64

add_mul(a, b, c) = muladd(b, c, a)

which rewrites a + b * c as b * c + a!!!! How annoying.

[odow]

Okay, this is now good to merge.