Unified platform method invocations

[ikurek]

This should resolve #359. In scope of this pull request:

• All parameters passed to platform instances are now wrapped in AblyMessage. This makes it easier to handle them on native side, and the communication from Dart to platform SDKs is more consistent
• Parameters of platform methods are now always a Map of values. Previously, these methods accepted Object which led to inconsistent behavior (for example, Crypto.generateRandomKey did not use a map with the generated TxCryptoGenerateRandomKey.keyLength value, but rather passed a raw int value)
• invoke method was altered to allow passing handle value besides the parameters, and invokeRaw was removed, since it became unnecessary.

[QuintinWillison]

I'll admit I've only given this a brief, light review - but on a skim read, nothing looks obviously out of place. Thanks. 😁