high `Serialization` time

[oscardssmith]

The following shows a case where the cycle detection in Serialization takes almost all of the time. (taken from https://discourse.julialang.org/t/serialize-is-prohibitively-slow-for-recursive-structs-what-am-i-missing/76276/12)

mutable struct MinimalNode
    childvalues::Union{NTuple{4, Int8}, NTuple{3, Int8}, NTuple{2, Int8}, NTuple{1, Int8}, Tuple{}};
    childnodes::Union{NTuple{4, MinimalNode}, NTuple{3, MinimalNode}, NTuple{2, MinimalNode}, NTuple{1, MinimalNode}, Tuple{}};
end

function addbranches!(mn::MinimalNode, aleft::Int64, bleft::Int64, parity::Int64, depth = 0)

    (depth == 12) && (return nothing);

    if parity == 1
        (aleft <= 1) && (return nothing);
        mn.childvalues = Tuple(1:aleft);
        mn.childnodes = Tuple([MinimalNode((), ()) for ii in 1:aleft]);
        aleft -= 1;
    else
        (bleft <= 1) && (return nothing);
        mn.childvalues = Tuple(1:bleft);
        mn.childnodes = Tuple([MinimalNode((), ()) for ii in 1:bleft]);
        bleft -= 1;
    end

    addbranches!.(mn.childnodes, aleft, bleft, 3-parity, depth + 1);
    
end

function buildfaketree()
    root = MinimalNode((), ());
    addbranches!(root, 4, 4, 1, 0);
    return root;
end


using Serialization;
MN = [buildfaketree() for ii in 1:10000];
@time serialize("test.jls", MN);
35 seconds

Notably, increasing the size of MN has a slightly super-linear effect on serialization time.