This does not include uses of "gen" by other libraries (for example
`rand::Rng::gen`).

This won't yet build. We'll fix the build issues in follow-up commits.

The default binding mode was changed to "ref" in rust 2021, and now in
rust 2024 it's an error to use the `ref` binding modifier when the
binding mode is already "ref".

For example:

```text
error: binding modifiers may only be written when the default binding mode is `move`
   --> lib/tcp/src/buffer.rs:347:27
    |
347 |             Segment::Data(ref data) => data.len().try_into().unwrap(),
    |                           ^^^ binding modifier not allowed under `ref` default binding mode
    |
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
```

```text
error[E0716]: temporary value dropped while borrowed
   --> lib/shim/src/signals.rs:119:13
    |
118 |           let action = tls_process_shmem::with(|process| *unsafe {
    |  ________________________________________________________-
119 | |             process.protected.borrow(&host_lock.root).signal_action(sig)
    | |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                  - temporary value is freed at the end of this statement
    | |             |
    | |             creates a temporary value which is freed while still in use
120 | |         });
    | |_________- borrow later used here
```

This is unrelated to the rust 2024 migration, but I noticed this so
decided to change it.

Lots of code churn here ¯\_(ツ)_/¯