OCB: Make primitive AES API explicit

Explicitly define the primitive AES API used by the internal OCB
implementation, and move it into its own namespace (ocb_aes). This will
ease future implementation changes.

Also make some style fixes to affected lines: Replace C-style casts
with C++-style casts, add some missing spaces in argument lists, and
remove some `inline` that the compiler will ignore.

Bug: https://github.com/mobile-shell/mosh/issues/1174