Investigate interfacing to a defined subset of PSA Crypto

[hug-dev]

The PSA Crypto API has some implementation-defined parts. For example, output size macros and various types need to be defined by the implementor.
The consequence of that are:

• the header file of the PSA Crypto implementation needs to be given as an input of this crate to generate the bindings
• bindings can not be pre-generated
• dynamic loading is impossible

A solution to investigate would be for us to define a subset PSA Crypto API where:

• all functions are defined but return PSA_ERROR_NOT_SUPPORTED if not implementend
• the key types and algorithms are limited to only the ones defined within the API (no extension)
• the output macro are implemented (they can be with the constraint above)
• the types are implemented, probably with void * pointer

We could implement all this in a header file, in this repo that we produce bindings from.

On the PSA Crypto implementors side, there will be a need to write a wrapper implementing our subset with the available functions.
In an ideal world, we could imagine a compilation option on the implementor project to produce a .a or .so which implements the subset instead of the original API. Then people will need to use that with Parsec.