APIs which do less JSON/Base64 wrapping

[richvdh]

The current APIs tend to take and return JSON objects with Base-64 encoding. In some applications this is a hindrance rather than a help; we should expose lower-level APIs which allow passing of raw strings.

Examples:

• olm_account_identity_keys generates a chunk of JSON - but apps end up having to parse the json to fish out the individual keys.

[Ralith]

Ideally, I'd like to see support for raw, completely unencoded data (i.e. not base64 strings, or any other text format) for all input and output, allowing encoding/decoding to eventually be factored out into external code, and skipped entirely when unneeded such as when persisting data locally. This could also improve reusability and ultimately reduce the amount of code to be maintained and audited within the olm library itself.

[spacekitteh]

Related is #5