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Union-split on `Expr`, `Symbol`, and `LineNumberNode` when hashing `Expr`s (#59378)

```julia-repl
x@fedora:~/.julia/dev/julia$ julia +pr59378
  o  | Version 1.13.0-DEV.1043 (2025-09-07)
 o o | lh/hash-expr-union-split/8a95cf82d3d (fork: 8 commits, 14 days)
julia> expr = Meta.parseall(read("base/show.jl", String));

julia> using ChairmarksExtras

julia> @btime expr hash
  395.753 μs (23358 allocs: 364.969 KiB)
0x8e1ffc47fe5dc80b

julia> @btime :(sin(x^2) + cos(x^2)) hash
  144.778 ns (15 allocs: 240 bytes)
0xc837adb769107933

julia> 
x@fedora:~/.julia/dev/julia$ julia +nightly
  o  | Version 1.13.0-DEV.1096 (2025-09-07)
 o o | Commit 8a384ab93e4 (0 days old master)
julia> expr = Meta.parseall(read("base/show.jl", String));

julia> using ChairmarksExtras

julia> @btime expr hash
  826.924 μs (22422 allocs: 350.344 KiB)
0xf4f9c5fc15a95298

julia> @btime :(sin(x^2) + cos(x^2)) hash
  275.557 ns (14 allocs: 224 bytes)
0xc837adb769107933
```

Notably, the hash of that big expressions changes between these versions because it contains global refs that have different `objectid`s on these two versions.

---------

Co-authored-by: Neven Sajko <4944410+nsajko@users.noreply.github.com>
Co-authored-by: Andy Dienes <51664769+adienes@users.noreply.github.com>

13 of 14 new or added lines in 2 files covered. (92.86%)

61541 of 79622 relevant lines covered (77.29%)

22781953.6 hits per line