Add namespacing for Core and PSA structures

[hug-dev]

With #30 in, there is a clear separation of Core and PSA structures with the Psa or psa_ prefix added everywhere.
A better idiomatic and scalable way at separating them would be to use namespacing.

In the operations, the Core and PSA files would be in their own folders.
In the interface, Core and PSA structures would be in their own module. There would be enumerations of PSA operation opcodes and Core operation opcodes with an Opcode enum of those enum at the top. Same for the ResponseStatus.

The Provide trait contains Core and PSA operations. One option could be to have two Provide traits: PsaProvide and CoreProvide.

This issue is mainly for discussion on those topics.

[ionut-arm]

It's worth considering if we want to go the "Mbed Crypto SE driver" way of having traits for different types of crypto operations - e.g. PsaAsymSign