…2484)

**Best reviewed on a commit-by-commit basis; in particular the `rerun
codegen` commit is nothing but generated code.**

This PR upgrades the Rust code generator to make use of actual
`TokenStream`s (i.e. `syn` & `quote`) rather than manipulating raw
strings; because what's coming in the following PRs puts us way past the
complexity threshold for using simple strings.

Interestingly, while `syn` & `quote` were obviously designed with Rust
in mind, they aren't specific to Rust so much as they are made to work
with bracket-base languages.
I.e. the Rust code generator becomes a nice starting point for a C or
C++ one.

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- #2484
- #2485 
- #2487 
- #2520 

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PR Build Summary: https://build.rerun.io/pr/2484

<!-- pr-link-docs:start -->
Docs preview: https://rerun.io/preview/cf52fbc/docs
Examples preview: https://rerun.io/preview/cf52fbc/examples
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**Best reviewed on a commit-by-commit basis; in particular the `rerun
codegen` commit is nothing but generated code.**

This PR updates (or rather, downdates?) the IDL definitions of
`Points2D` and its components to make sure they match the legacy ones
(e.g. `Point2D` is back to being a `struct { x, y }` rather than a
`[f32; 2]`).

We'll revert that once we're done with the backporting/migration and get
rid of the legacy stuff.

UPDATE: This now also includes the `attr.rerun.legacy_fqname` attribute,
which does what you think it does.

---

- #2484
- #2485 
- #2487 
- #2520 

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<!-- This line will get updated when the PR build summary job finishes.
-->
PR Build Summary: https://build.rerun.io/pr/2485

<!-- pr-link-docs:start -->
Docs preview: https://rerun.io/preview/41e265f/docs
Examples preview: https://rerun.io/preview/41e265f/examples
<!-- pr-link-docs:end -->