std/crypto: use finer-grained error sets in function signatures
Returning the `crypto.Error` error set for all crypto operations
was very convenient to ensure that errors were used consistently,
and to avoid having multiple error names for the same thing.
The flipside is that callers were forced to always handle all
possible errors, even those that could never be returned by a
function.
This PR makes all functions return union sets of the actual errors
they can return.
The error sets themselves are all limited to a single error.
Larger sets are useful for platform-specific APIs, but we don't have
any of these in `std/crypto`, and I couldn't find any meaningful way
to build larger sets.
We currently have ciphers optimized for performance, for
compatibility, for size and for specific CPUs.
However we lack a class of ciphers that is becoming increasingly
important, as Zig is being used for embedded systems, but also as
hardware-level side channels keep being found on (Intel) CPUs.
Here is ISAPv2, a construction specifically designed for resilience
against leakage and fault attacks.
ISAPv2 is obviously not optimized for performance, but can be an
option for highly sensitive data, when the runtime environment cannot
be trusted.
