inline functions instead of normal fun

[lkishor]

Thanks, for this awesome library on the top what Rx is 👍

fun Disposable.addTo(compositeDisposable: CompositeDisposable): Disposable = apply { compositeDisposable.add(this) }

Can this fun be inlined, and probably other functions with such small operations.
This would remove any overhead of this function call.

[vpriscan]

@thomasnield should we make all functions inline? This will especially be beneficial to android users as it will not increase methods count.