Reorganize sources for future work

[lorentey]

This is a standalone PR for some of the code organization changes in #94. This makes it slightly easier to navigate the project, and prepares for the introduction of struct Atomic later.

Mostly NFC, although I did slip in a drive-by @inline(__always) → @_transparent adjustment. This PR also embraces _pointerBitWidth in Swift 5.9+, instead of unwieldy and fragile arch disjunctions.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test