Inline trait methods in derive-generated code

## Status

- [x] #341
- [ ] Deny `clippy::missing_inline_in_public_items` in derive-generated code

## zerocopy-derive

We don't care about inline attributes for zerocopy-derive itself, but we _do_ care for code _emitted_ by zerocopy-derive. We need to figure out a way to either modify zerocopy-derive's output or modify our zerocopy-derive tests so that, when running zerocopy-derive tests, missing `#[inline]` attributes generate warnings or errors. It might be possible to use the `clippy::missing_inline_in_public_items` lint for this, but I'm not sure.

## zerocopy

~~Many zerocopy trait methods contain very little logic or no logic at all, but are currently not marked with any `inline` attribute, and so cannot be inlined across a crate boundary. We should change this.~~

`#[inline]` attributes are now enforced by Clippy as of #341.

## Mentoring instructions

*Interested in contributing? See our [contributing guide](https://github.com/google/zerocopy/discussions/1318).*

Figure out how we can ensure that the `clippy::missing_inline_in_public_items` lint is enforced (at least during testing) in code emitted by zerocopy-derive.