Remove usage of try_into().unwrap() where feasible

[xoloki]

Early versions of wsts used usize types anywhere that the code would deal with the size of memory. But this is problematic when sending data over the network, since usize is a platform dependent type.

So at some point we changed all public integer types to be fixed size, i.e. u32 or u64. This required sprinkling as usize casts where needed. However, @jferrant discovered that as usize casts will fail silently if the cast value is improperly sized.

As a result, we now call try_into().unwrap() to handle all casts between u32/u64 and usize. The logic was that while these can fail, they are extremely unlikely to do so, and if they do it's likely a result of a misconfiguration that should be noticed and fixed.

However, since this is library code, it is bad form to ever panic when an error could be returned instead. So this issue will track work to remove these unwrap calls whenever feasible.