internal: use std as extern crate

[Person-93]

This PR is to prepare for no_std support.

• I've made the main crate no_std and added extern crate std instead.
• I've also added a new top-level mod platform to hold all the upcoming std replacements. As of now, it just has a prelude which contains
  • Things from the alloc crate people generate use from std's prelude
  • the eprintln macro (we will need to find a replacement for this)
• I've added import crate::platform::prelude::* to every file that uses things from the standard prelude.
• The test_utils file could not import the prelude so it had to import directly from the alloc crate.
• in py_format macro: replace use of ToOwned with Clone

ETA: It looks like a large PR because of the number of files changed, but most of that is just importing the new prelude into the files that need it. There are only four files that have more changes than that.

[davidhewitt]

Let's merge this to unblock proceeding on this work.

[davidhewitt]

I can fix the conflicts after #6056 merges.

[codspeed-hq]

Merging this PR will not alter performance

✅ 126 untouched benchmarks

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_{Comparing Person-93:extern_std (1c47f3c) with main (1df2875)}

[Person-93]

@davidhewitt #6056 is not ready to be merged, can we proceed with this one?

[davidhewitt]

Sure thing

[Person-93]

The last failure was github's fault. https://github.com/PyO3/pyo3/actions/runs/27917137210/job/82604843356#step:8:36

[Person-93]

Yay!!!