Make attiny_hal::pins! and atmega_hal::pins! feature-agnostic

[innermatrix]

This removes the feature-specific (and therefore MCU-specific) pins! macro and replaces it with a feature-agnostic pins! macro + an MCU-specific wrapper function.

This is initial work to enable a refractor that will switch all crates in arduino-hal to use additive features, as is the recommended best practice. See #602 for more context.

Testing:

• Verified that cargo build -p arduino-hal --features <board> succeeds for all boards
• Verified that cargo build --bins succeeds for every example/*

Using stacked PRs — see innermatrix#1 (comment) for a preview of the follow-on work in progress.