Speed up with MutableArithmetics

[blegat]

Before

BenchmarkTools.Trial: 10000 samples with 4 evaluations.
 Range (min … max):   8.011 μs …  40.041 ms  ┊ GC (min … max):  0.00% … 99.93%
 Time  (median):      8.852 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   15.422 μs ± 401.401 μs  ┊ GC (mean ± σ):  34.24% ±  4.36%

   ▃▅▇██▇▆▅▄▃▂▂▁▂▁▁▁▁                       ▁▂▃▄▄▄▄▄▃▃▂▂▁▁     ▃
  ▇█████████████████████▇▇█▆▇▅▆▁▅▁▁▄▃▁▁▃▁▃▆▇███████████████▇▇▆ █
  8.01 μs       Histogram: log(frequency) by time      16.6 μs <

 Memory estimate: 31.62 KiB, allocs estimate: 597.

After

BenchmarkTools.Trial: 10000 samples with 7 evaluations.
 Range (min … max):  4.033 μs …  17.567 ms  ┊ GC (min … max):  0.00% … 99.92%
 Time  (median):     4.293 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   7.163 μs ± 175.969 μs  ┊ GC (mean ± σ):  30.43% ±  3.89%

  ▁▆██▇▅▃▁▁▁▁▁                           ▁▂▂▃▃▃▄▃▃▃▃▂▁        ▂
  ██████████████▇▆▆▆▆▆▅▅▄▄▂▄▄▅▅▇▇▆▄▃▂▄▆▆███████████████▇▇▇▅▅▅ █
  4.03 μs      Histogram: log(frequency) by time      8.13 μs <

 Memory estimate: 14.02 KiB, allocs estimate: 254.

[codecov]

Codecov Report

Attention: Patch coverage is 88.44221% with 23 lines in your changes missing coverage. Please review.

Project coverage is 88.37%. Comparing base (8c131a6) to head (0350c1d).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/elements.jl	62.06%	11 Missing ⚠️
src/repelemop.jl	76.47%	8 Missing ⚠️
src/comp.jl	88.88%	3 Missing ⚠️
src/incidence.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
- Coverage   88.98%   88.37%   -0.62%     
==========================================
  Files          37       37              
  Lines        3205     3226      +21     
==========================================
- Hits         2852     2851       -1     
- Misses        353      375      +22     

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