Fix unary minus operators in rewrite

[odow]

Closes #323

[odow]

The benchmark in #323 becomes:

julia> using JuMP

julia> function test1()
           model = Model()
           N = 1_000_000
           @variable(model, x[1:N])
           GC.gc()
           GC.gc()
           @time for i in 1:5
               @expression(model, sum(-1.0 * x[i] for i in 1:N))
           end
           GC.gc()
           GC.gc()
           @time for i in 1:5
               @expression(model, sum(-x[i] for i in 1:N))
           end
           GC.gc()
           GC.gc()
           @time for i in 1:5
               @expression(model, -sum(x[i] for i in 1:N))
           end
           return
       end
test1 (generic function with 1 method)

julia> test1()
  1.047293 seconds (235 allocations: 223.388 MiB, 1.38% gc time)
  1.011328 seconds (235 allocations: 223.388 MiB, 1.35% gc time)
  1.508724 seconds (240 allocations: 223.389 MiB, 0.91% gc time)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.32%. Comparing base (1415163) to head (ac973fc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
+ Coverage   90.30%   90.32%   +0.02%     
==========================================
  Files          22       22              
  Lines        2238     2253      +15     
==========================================
+ Hits         2021     2035      +14     
- Misses        217      218       +1     

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