aes: add #[inline] attributes for KeyInit::new impls

[newpavlov]

Fixes #385

[tarcieri]

This has no effect on #385:

   Compiling aes v0.8.3 (https://github.com/RustCrypto/block-ciphers.git?branch=aes_inline#a8bbd2f9)
[...]
[stack]: 8388608 bytes
4001893a10  40 40 40 40 40 40 40 40  40 40 40 40 40 40 40 40   !!!!!!!!!!!!!!!!
4001893a20  40 40 40 40 40 40 40 40  40 40 40 40 40 40 40 40   !!!!!!!!!!!!!!!!
4001893b10  40 40 40 40 40 40 40 40  40 40 40 40 40 40 40 40   !!!!!!!!!!!!!!!!
4001893b20  40 40 40 40 40 40 40 40  40 40 40 40 40 40 40 40   !!!!!!!!!!!!!!!!
4001893dc0  40 40 40 40 40 40 40 40  40 40 40 40 40 40 40 40   !!!!!!!!!!!!!!!!
[...]

FWIW I tried something similar in https://github.com/RustCrypto/block-ciphers/tree/aes/armv8-keep-key-schedule-off-stack

[tarcieri]

That said, this is probably still worth merging.