aes: allow detecting whether hardware acceleration is available

[SludgePhD]

It is often useful to be able to detect when block ciphers can be hardware accelerated, because the performance difference between a software and hardware implementation can be significant (especially with AES).

Detecting the availability of hardware acceleration in the crates here would allow applications to better choose an algorithm, or to log a warning (or collect telemetry) noting the possible performance degradation.

[tarcieri]

We could potentially expose the InitToken which stores the detection result the crate itself uses in the public API, though its currently only available on select targets.

That puts cpufeatures in public API which prevents breaking upgrades, but really even in the last release the breaking changes were minimal so I don't think we have any major breaking upgrades planned.

Otherwise a small toplevel free function that's always available and returns a bool could work too.

[SludgePhD]

Otherwise a small toplevel free function that's always available and returns a bool could work too.

This seems preferable, and it would be obvious how to use it.

Not sure why I'd want some internal type instead.

[tarcieri]

It’s a ZST which caches the result. The free function will call the underlying CPU feature detection every time, which on some platforms incurs a system call

[newpavlov]

Otherwise a small toplevel free function that's always available and returns a bool could work too.

I would prefer to do this.