Bsn macros fuzzing

[mockersf]

Objective

• Fuzz bsn! and bsn_list!

Solution

• use https://github.com/rust-fuzz/cargo-fuzz
• add fuzz targets for bsn! and bsn_list!

Built on top of #23682 as I was already on that branch to reuse stuff

Testing

• CI should run the fuzz tests
• for f in fuzz_targets/*.rs; do; cargo +nightly fuzz run $(basename "$f" .rs) -- -runs=1000; done

[mockersf]

found panics:

• bsn_list![C((#u))]
• bsn!(A { x: (#u) })

with errors:

error: proc macro panicked
    |
115 |     bsn_list![C((#u))];
    |     ^^^^^^^^^^^^^^^^^^
    |
    = help: message: internal error: entered unreachable code

error: proc macro panicked
    |
116 |     bsn!(A { x: (#u) });
    |     ^^^^^^^^^^^^^^^^^^^
    |
    = help: message: internal error: entered unreachable code

[laundmo]

found panics:

* `bsn_list![C((#u))]`

* `bsn!(A { x: (#u) })`

I don't think this PR has the changes from #24174 right? That'll likely affect those snippets, as it changes how #name works slightly

[mockersf]

I used Opus 5.5 to update the corpus in commit 774c48b