Change type for fields denoting sizes

[ionut-arm]

Most fields denoting sizes (e.g. of keys) are marked as usize at the moment. To make it less ambiguous, the ones representing sizes in bits should be set to u16, u32 or other types unrelated to word size on current platform (as the size itself is not related to the memory of the platform)

[hug-dev]

Is it still needed? We went back on this idea for PSA Crypto I believe

[Superhepper]

This is still needed and should perhaps be added as coding guideline. Because I still do this mistake of just using usize.

[ionut-arm]

should perhaps be added as coding guideline

While I'd love to have coding guidelines and be very explicit about how to handle various cases, at least from my experience it's difficult to actually keep them all in mind when coding (or when reviewing code). Maybe we should write some guidelines up nonetheless, so that we have something to fall back onto when debating how things should be implemented in a PR.

[ionut-arm]

Is it still needed? We went back on this idea for PSA Crypto I believe

We went back on this idea for PSA Crypto because that spec explicitly uses usize for those sort of types, but I'm still not convinced it's as clear-cut