Fix slow show(grid)

[KnutAM]

The use of repr on every cell in #570 was very slow.
With this PR, there is still performance regression for mixed grids (factor 3), but still ok (in my opinion).

@fredrikekre: Is there a better way to get each type in the union? In that case the suggestion below might make sense, even though I don't think it matters that much...

[codecov-commenter]

Codecov Report

Base: 92.79% // Head: 92.77% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (da02f53) compared to base (930887e).
Patch coverage: 66.66% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #599      +/-   ##
==========================================
- Coverage   92.79%   92.77%   -0.03%     
==========================================
  Files          28       28              
  Lines        4222     4224       +2     
==========================================
+ Hits         3918     3919       +1     
- Misses        304      305       +1     

Impacted Files	Coverage Δ
src/Grid/grid.jl	87.16% <66.66%> (-0.23%)	⬇️

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[fredrikekre]

If you create the Set with types instead of strings it seems pretty ok:

julia> @time sort!(collect(Set(repr(typeof(x)) for x in grid.cells)))
110.462024 seconds (44.03 M allocations: 17.203 GiB, 1.71% gc time, 0.03% compilation time)
1-element Vector{String}:
 "Triangle"

julia> @time collect(Set(typeof(x) for x in grid.cells))
  0.378749 seconds (247.76 k allocations: 51.872 MiB, 5.09% gc time, 35.21% compilation time)
1-element Vector{DataType}:
 Triangle (alias for Cell{2, 3, 3})

[KnutAM]

Exactly, that's where I got the 3x (actually 5x for this timing) slowdown from though:

oldversion(grid) = sort!(collect(Set(celltypes[typeof(x)] for x in grid.cells)))
newversion(grid) = sort!(repr.(Set(typeof(x) for x in grid.cells)))
masterversion(grid) = sort!(collect(Set(repr(typeof(x)) for x in grid.cells)))
grid = generate_grid(Triangle, (100, 100));
@time newversion(grid);
  0.002467 seconds (34 allocations: 298.586 KiB)
 @time oldversion(grid);
  0.000490 seconds (9 allocations: 288.680 KiB)
@time masterversion(grid);
  0.921047 seconds (480.01 k allocations: 192.543 MiB, 3.66% gc time

(Timing after a first run for compilation and I have defined the const celltypes=Dict from the previous Ferrite version)