Split rslib into smaller crates to speed up incremental builds

[user1823]

Problem

The anki crate (rslib/) is a very large crate. Even a small change to any file in rslib/ forces a full recompilation of the entire crate, resulting in very long build times.

Proposed Solution

Split rslib into smaller, feature-specific sub-crates (e.g. anki_scheduler, anki_search, anki_sync, anki_storage). This way only the sub-crate containing the changed file needs recompiling.

There is already a precedent for this in the form of rslib/i18n, rslib/proto subcrates.