External Transactron

[tilk]

This PR finally extracts Transactron to external repo, https://github.com/kuznia-rdzeni/transactron. Depends on #754.

In the same time, the amaranth-stubs submodule is removed and replaced by an entry in requirements.txt. My reasons are:

1. Installing local packages is not great when pyproject.toml is used. I'd like to move Coreblocks to pyproject.toml - I already did that in transactron.
2. Workflows can be simplified, as the submodule doesn't need to be fetched before dependency installation.
3. There are no real negative consequences for development - the local amaranth-stubs version needed to be installed using pip after each change anyway.

[piotro888]

Would it make sense to rebase transactron repository on top of something like git-filter-repo (note: invocation is destructive!) with --path transactron/ --path test/transactron/ --path-rename test/transactron/:test/ --path coreblocks/transactions --replace-message <replace file with "#==>https://github.com/kuznia-rdzeni/coreblocks/pull/"> to keep easier to get history with valid PR references in one place?

[piotro888]

Coreblocks README needs updating; and maybe docs/transactions.md should be preserved, but only with url to Transactron gh-pages docs?

[tilk]

Would it make sense to rebase transactron repository on top of something like git-filter-repo (note: invocation is destructive!) with --path transactron/ --path test/transactron/ --path-rename test/transactron/:test/ --path coreblocks/transactions --replace-message <replace file with "#==>https://github.com/kuznia-rdzeni/coreblocks/pull/"> to keep easier to get history with valid PR references in one place?

I just did that, but it unfortunately invalidated all active PRs in Transactron repo.