⚡️ Add destructive_add! for variable + variable

[blegat]

The result of speed.jl on master:

P-Median(100 facilities, 100 customers, 5000 locations) benchmark:
BenchmarkTools.Trial: 
  memory estimate:  1017.98 MiB
  allocs estimate:  16531537
  --------------
  minimum time:     2.028 s (44.50% GC)
  median time:      2.303 s (52.62% GC)
  mean time:        2.320 s (52.55% GC)
  maximum time:     2.630 s (58.70% GC)
  --------------
  samples:          3
  evals/sample:     1
Cont5(n=500) benchmark:
BenchmarkTools.Trial: 
  memory estimate:  421.65 MiB
  allocs estimate:  7545407
  --------------
  minimum time:     915.800 ms (41.56% GC)
  median time:      1.003 s (43.59% GC)
  mean time:        999.647 ms (43.82% GC)
  maximum time:     1.078 s (45.62% GC)
  --------------
  samples:          6
  evals/sample:     1

With this PR

P-Median(100 facilities, 100 customers, 5000 locations) benchmark:
BenchmarkTools.Trial: 
  memory estimate:  735.64 MiB
  allocs estimate:  13030837
  --------------
  minimum time:     1.416 s (35.18% GC)
  median time:      2.219 s (58.35% GC)
  mean time:        2.012 s (53.49% GC)
  maximum time:     2.401 s (59.80% GC)
  --------------
  samples:          3
  evals/sample:     1
Cont5(n=500) benchmark:
BenchmarkTools.Trial: 
  memory estimate:  421.08 MiB
  allocs estimate:  7538407
  --------------
  minimum time:     912.507 ms (41.36% GC)
  median time:      971.512 ms (43.71% GC)
  mean time:        971.481 ms (43.99% GC)
  maximum time:     1.038 s (46.60% GC)
  --------------
  samples:          6
  evals/sample:     1

Related to #1403

[codecov]

Codecov Report

Merging #1636 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1636   +/-   ##
=======================================
  Coverage   90.86%   90.86%           
=======================================
  Files          28       28           
  Lines        3744     3744           
=======================================
  Hits         3402     3402           
  Misses        342      342

Impacted Files	Coverage Δ
src/parse_expr.jl	94.9% <100%> (+0.02%)	⬆️
src/macros.jl	89.89% <0%> (-0.02%)	⬇️

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