Use sorted OrderedSet in FieldHandler

[fredrikekre]

This changes FieldHandler.cellset to be a sorted OrderedSet instead of a Set. This ensures that loops over sub-domains are done in ascending cell order.

Since e.g. cells, node coordinates, and dofs are stored in ascending cell order this gives a significant performance boost to loops over sub-domains, i.e. assembly-style loops. In particular, this removes the performance gap between MixedDofHandler and DofHandler in the create_sparsity_pattern benchmark in #629.

This is a minimal/initial step towards #625 that can be done before the DofHandler merge and rework of FieldHandler/SubDofHandler.

[fredrikekre]

Perhaps BitSet would be an alternative here, actually. That would store at maximum getncells(grid) / 64 (if the cell IDs in a subdomain are randomly distributed in 1:getncells(grid)) and at minimum length(set) / 64 (if each subset contain consecutively numbered cells, 1:10, 11:20, ...) UInt64 per sub-domain. Compared to Set, which stores roughly length(set) * 3 / 2 (I believe dict buffers should be filled up to at most 2/3), this would become worse if your set is randomly numbered and you have more than 64 * 3 / 2 ≈ 100 subdomains (and strictly better if you number your cells according to the subdomains, which I think is common anyway). I don't think this matters much though, this storage is nothing compared to dof storage, for example. Maybe it is easier to use BitSet since that is built in :)

[KnutAM]

Realized today that at least for threading, the ordered set must be collected: https://julialang.slack.com/archives/C6SMTHQ3T/p1683401329225109.
Copied example from that thread:

using OrderedCollections
vec = [1,3,5];
v = zeros(Int, 5);
oset = OrderedSet(vec);
Threads.@threads :static for i in vec
    v[i] = Threads.threadid()
end
Threads.@threads :static for i in oset
    v[i] = Threads.threadid()
end

ERROR: TaskFailedException
Stacktrace:
 [1] wait
   @ .\task.jl:345 [inlined]
 [2] threading_run(fun::var"#45#threadsfor_fun#5"{var"#45#threadsfor_fun#4#6"{OrderedSet{Int64}}}, static::Bool)
   @ Base.Threads .\threadingconstructs.jl:38
 [3] top-level scope
   @ .\threadingconstructs.jl:93

    nested task error: MethodError: no method matching firstindex(::OrderedSet{Int64})
    Closest candidates are:
      firstindex(::Any, ::Any) at abstractarray.jl:402
      firstindex(::Tuple) at tuple.jl:25
      firstindex(::Pair) at pair.jl:50
      ...
    Stacktrace:
     [1] #45#threadsfor_fun#4
       @ .\threadingconstructs.jl:69 [inlined]
     [2] #45#threadsfor_fun
       @ .\threadingconstructs.jl:51 [inlined]
     [3] (::Base.Threads.var"#1#2"{var"#45#threadsfor_fun#5"{var"#45#threadsfor_fun#4#6"{OrderedSet{Int64}}}, Int64})()
       @ Base.Threads .\threadingconstructs.jl:30