Re-organize using a proc-macro to support more devices

[TethysSvensson]

Summary

This is a continuation an continuation of the work I did in #716 and #719. It uses the same basic approach as #719, but I changed the macros in response to the feedback I got on that PR.

Since this is quite a large PR, I have split it up by commits, to make it easier to review.

A quick overview of what this PR contains:

• It adds a new proc-macro crate, which replaces all of the many-many features in the Cargo.toml
• The primary macro of that crate is #[hal_cfg()], but it also contains a helper macro to allow using #[hal_cfg()] in more places, along with two specialized macros for creating conditional docstrings and conditional modules.
• A lot of doc-comments are currently conditional depending on feature flags. This PR removes most of these, as they cannot easily be supported by the new proc-macro.
• A lot of functionality were split up first by architecture in the thumbv6m and thumbv7em directories, and then by peripheral. This PR moves these files, so they are organized first by peripheral and then by architecture.
• The code for reset_cause, serial_number and watchdog were sufficiently similar that I decided to merge the implementations.
• It does not create a workspace, since I got weird crate unification errors when I did.

This PR also includes a change in sercom/pad/impl_pad_thumbv6m.rs should arguably be split into a separate PR. The is that while converting the pad_table, I found a mistake in the table and did not feel like carrying that mistake over.

However I wanted to review the rest of the table for mistakes as well, which I found almost impossible to do unless I split up the tables by datasheet.

If you want me to split out this change in a separate PR, please let me know whether you prefer me to carry over the bug in the conversion, or if you prefer to fix the bug before merging this PR.

Checklist

• CHANGELOG.md for the BSP or HAL updated
• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.

[jbeaurivage]

Thanks a lot for this PR @TethysSvensson. After reading through all your changes, I can't seem to find anything I disagree with. I'd just be interested in knowing what the bugs were in the SERCOM pads tables?

[TethysSvensson]

I'd just be interested in knowing what the bugs were in the SERCOM pads tables?

I don't remember exactly how I first noticed it. I believe that the samd11 tables might have been different enough that I somehow spottet it?

In any case, the new tables are definitely different from the old ones, and I did compare them very carefully from the manual at some point, so I believe the new ones are right.

It was a while ago that I verified, so perhaps I should take another look to see if any mistakes snuck back in 🤔

[TethysSvensson]

After re-checking, I didn't find a bug, as the new tables were mostly equivalent to the old ones. The only changes I found where these added pads to d11:

PA06
PA07
PA10
PA11
PA14/D
PA15/D
PA16
PA17
PA22
PA23
PA24/D
PA25/D
PA30/D
PA31/D

I think what I was as a "bug" was something like the PA14/D, where the pads are different between d11 and d21, but I misread the #[cfg()] to assume that it also applied to the d11.

If you want me to re-combine the tables I can do that.

[jbeaurivage]

Nope it sounds good to me, I kinda like splitting the tables, it makes them more straightforward to understand. Anything else you want to review/discuss before we merge this?

[TethysSvensson]

I'm good with merging now. As I said I haven't done any automated backwards compatible testing.

I don't know when I'll have time to do so either, so if you want that done before merging I'd love some help.

[jbeaurivage]

Well, we rarely release patch updates anyway, so the next release will likely be a minor anyways, with no expectation of backwards compatibility. LGTM, thanks for the hard work!