Assume global state for the board API

[ia0]

Issues with the current design:

• There is some lifetime management involved. In particular, the Types trait is needed to work around that.
• Some APIs don't need a global state, but they still take &mut self. For example SHA-256 and HMAC-SHA-256 just have their own context and don't need a global state. In particular, it is not possible to provide a state-less implementation (e.g. of RustCrypto traits) on top of the board API, which is needed when software implementation must be "chained" (e.g. software HMAC-SHA-256 on top of hardware SHA-256).

The proposal is to put the burden of handling global state (when needed) to the board implementer. Maybe helpers could be provided. We should also then be able to reuse existing state-less APIs like RustCrypto.

A possible alternative would be to decide when the board API should have a global state. But this is assuming something on all implementations, and those that needs a state when the API is state-less would still need to handle their global state like for the other proposal above. So a more uniform approach is preferred.