Encryption part 1: Restructure SDK modules

[simolus3]

Currently, :core has build logic ensuring a copy of SQLite is bundled with all apps depending on it. That's usually what we want because the SQLite build from operating systems is either unavailable (Android) or outdated / missing extension support (Apple).

However, it also means that the SQLite library can't be customized. This is an issue for users wanting to use say SQLCipher. As a first step towards that, this:

1. Introduces the :common (really unfortunate that :core was taken :D) project containg virtually all sources and tests that currently live in :core. The only difference is that this project doesn't depend on sqlite-bundled or the :static-sqlite-driver.
2. Refactors :core to mostly be an empty shell depending on :common and the bundled SQLite dependencies.
3. Introduces a test helper so that we can write tests in both SDKs. The test helper allows :common to depend on :core and bundled SQLite in its tests only.

The idea is that users not wanting to depend on the regular SQLite build could customize things by depending on :common instead.