Use better AES/SHA implementations

[richvdh]

At present, libolm uses Brad Conte's portable implementations [1] of SHA-256 and AES-256. These have the advantage of being plain C and therefore work out-of-the-box under emscripten; however they are not side-channel resistant.

I'm not aware of any portable implementations of AES which are resistant to side-channel attacks - indeed it is very difficult to create a constant-time implementation without access to machine-level detail. The correct fix to this is therefore to use different implementations in different environments - for example, we could link against openssl for a C library, and use webcrypto in the browser.

It's also likely that doing so would bring performance improvements.

The main difficulty is that it significantly changes data flows: for example, webcrypto provides an asynchronous interface, which means that the whole olm interface would need to be altered to be asynchronous, at least under emscripten.

[spacekitteh]

libsodium is a pretty good implementation of various crypto primitives... I think it only does AES-GCM, not AES-CBC however :/