Compute normal_form with mpolys

[SoongNoonien]

As proposed in #250 I tried to perform some computations with mpolys. This is the result when the normal_form computation is done with mpolys instead of upolys.

With current master (example from PR #238):

julia> @time scalar_test(g)
 76.226584 seconds (236.21 M allocations: 24.637 GiB, 12.26% gc time, 6.17% compilation time)

julia> @time scalar_test(g)
 70.591532 seconds (233.28 M allocations: 24.491 GiB, 12.26% gc time)

With this PR:

julia> @time scalar_test(g)
 74.355277 seconds (235.96 M allocations: 24.629 GiB, 12.84% gc time, 6.04% compilation time)

julia> @time scalar_test(g)
 69.110084 seconds (233.02 M allocations: 24.484 GiB, 11.68% gc time)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.37%. Comparing base (dca1ec2) to head (1894be0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   95.96%   95.37%   -0.59%     
==========================================
  Files          12       12              
  Lines         843      844       +1     
==========================================
- Hits          809      805       -4     
- Misses         34       39       +5     

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[fingolfin]

Before:

julia> using GenericCharacterTables, Chairmarks

julia> g=generic_character_table("3D4.0");

julia> @b scalar_product(g[3],g[7])
3.098 ms (75572 allocs: 3.831 MiB)

julia> @b scalar_product(g[20],g[21])
77.750 ms (995574 allocs: 92.981 MiB, 13.43% gc time)

After:

julia> using GenericCharacterTables, Chairmarks

julia> g=generic_character_table("3D4.0");

julia> @b scalar_product(g[3],g[7])
2.964 ms (75351 allocs: 3.824 MiB)

julia> @b scalar_product(g[20],g[21])
79.332 ms (993938 allocs: 92.931 MiB, 11.90% gc time)

So it is not much but there. Undraft and merge? We can add more improvements later.