Reimplemented context switching between security domains

[wojciechozga]

This PR brings major revision of the context switch implementation. It introduces:

• Lightweight context switches (in assembly) between hypervisor&security monitor AND confidential vm&security monitor
• A heavy context switch (implemented in Rust) that stores CSRs of one domain and reloads CSR of the next security domain.

Additionally, we got rid of the riscv crate in favor of a smaller CSR manipulation implementation.

Further work is needed to make sure that all relevant CSRs are stored/loaded during context switches.