Thanks to Dominic Fischer for spotting.

instead of having a static one, as that could end up taking up a
lot of memory if your app keeps olm sessions hanging about.

As a way to dump the state of an olm session, ie. the chain indicies,
so we can debug why olm sessions break and get out of sync.

because older versions of emscripten don't support the length argument to
UTF8ToString.

The Pointer_stringify() function is deprecated and has a couple of
issues because it tries to guess the encoding of the buffer. In some
cases it can ignore the length parameter which could end up in
inconsistencies.

Switch to UTF8toString() that takes a length parameter and respects,
that way we don't need to allocate an additional byte for a NULL byte.

Change interface to allow the app to get the private part of the
key and instantiate a decryption object from just the private part
of the key.

Changes the function generating a key from random bytes to be
initialising a key with a private key (because it's exactly the
same thing). Exports & imports private key parts as ArrayBuffer at
JS level rather than base64 assuming we are moving that way in
general.

Don't think this matters since there's no PkEncryption /
PkDecryption object being passed, but for the sake of consistency

So we only init the library once.

Change the interface again, hopefully this time a bit more normal.
Now we wrap the emscripten module completely and just expose the
high level objects.

The olm library export is now imported as normal (ie. returns
a module rather than a function returning a module) but has an
`init` method which *must* be called. This returns a promise
which resolves when the module is ready. It also rejects if the
module failed to set up, unlike before (and unlike the
promise-not-a-promise that emscripten returns).

Generally catch failures to init the module.

The closure compiler was just renaming the variable so it never
would have picked them up. Make it an extern so it knows what to do.

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).