refactor: consolidate modules across the project

[dangusev]

This PR addresses two code structure issues:

1. The project has flat structure, which makes it harder to navigate and comprehend.
2. mod.rs files contain actual logic in many places

Changes":

1. Consolidated modules together in logical groups to add structure to the lib.
2. Moved code out of mod.rs (except rtc/join/mod.rs, it's too big and needs a separate PR)