Quite a lot going on in this PR:
* Updates to support recent emscripten, switching to WASM which is now the default
* Use emscripten's MODULARIZE option rather than wrapping it ourself, since doing
so in pre-post js doesn't work anymore.
* Most changes are moving the emscripten runtime functions to top-level
calls rather than in the Module object.
* Get rid of duplicated NULL_BYTE_PADDING_LENGTH
* Fix ciphertext_length used without being declared
* Fix things that caused the closure compiler to error, eg. using
OLM_OPTIONS without a declaration.
* Wait until module is inited to do OLM_ERROR = olm_error()
The main BREAKING CHANGE here is that the module now needs to initialise
asyncronously (because it has to load the wasm file). require()ing olm
now gives a function which needs to be called to create an instance.
The resulting object has a promise-like then() method that can be used
to detect when the module is ready. (We could use MODULARIZE_INSTANCE
to return the module directly as before, rather than the function,
but then we don't get the .then() method).
