Fixing PSA key slot exhaustion problem

[ElsaLopez133]

Each p256_ecdh call imports a key into a PSA key slot, and if those slots aren't being freed after use, it runs out and PSA returns InsufficientMemory.

[chrysn]

We're .unwrap()'ing the PSA results in other places as well. I don't think this should be an exception:

• Either something goes wrong in terms of "the item you want to delete does not exist": then ignoring the error means we're ignoring an error in how we use PSA.
• Or something goes wrong in terms of "oups your PSA coprocessor got unplugged": then this might just as well happen in the line above, and I'd prefer consistency.

[chrysn]

Suggested change

	unsafe { key_management::destroy(my_key).unwrap() };
	// SAFETY: The function demands that the Id is not used while destroyed.
	// We did not hand out the Id `my_key` in the last few lines, so we can destroy it.
	unsafe { key_management::destroy(my_key).unwrap() };

(Also, I feel reaffirmed in doing embedded-cal rather than fixing psa_crypto…)