split repo into the `core` wallet and zephyr-specific parts

[pcppcp]

this repo will eventually split into two repositories.
The core repository will include a minimal and portable library that performs basic crypto functions.
The rest will remain in zephyr-wallet repo - this is the current app we build for zephyr targets.

Why we need this?

To support multiple hw architectures (i.e. a ESP32), we need to carefully separate the core wallet functionality. This new core library should also make it easy to use hw crypto functions if supported by the MCU.

• anything that's not related to zephyr should go into a separate core repo
• the core library is included as a submodule in the zephyr-wallet repo
• the core repo will use CI/googletest to check basic functionality