Serialization: fast path for serialize(_, ::Union)

[adienes]

fixes #58007

PR:

julia> using Serialization, BenchmarkTools, LinearAlgebra

julia> const sig = Base.unwrap_unionall(methods(LinearAlgebra.herk_wrapper!).ms[1].sig);

julia> const sig_bytes = (io = IOBuffer(); serialize(io, sig); take!(io));

julia> @benchmark serialize(IOBuffer(), $sig)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  141.166 μs …  2.343 ms  ┊ GC (min … max): 0.00% … 92.20%
 Time  (median):     142.625 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):   144.458 μs ± 22.477 μs  ┊ GC (mean ± σ):  0.15% ±  0.92%

 Memory estimate: 13.95 KiB, allocs estimate: 253.

julia> @benchmark deserialize(IOBuffer($sig_bytes))
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):   97.292 μs …  7.686 ms  ┊ GC (min … max): 0.00% … 97.60%
 Time  (median):      99.083 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):   102.759 μs ± 99.222 μs  ┊ GC (mean ± σ):  1.58% ±  1.68%

 Memory estimate: 34.73 KiB, allocs estimate: 744.

master:

julia> const sig_bytes = (io = IOBuffer(); serialize(io, sig); take!(io))
# wait 10 minutes then give up
^C^C^C^C^C^CWARNING: Force throwing a SIGINT
ERROR: InterruptException: