API: Change celt_encode and celt_encode_float so that they take an optional...

API: Change celt_encode and celt_encode_float so that they take an optional synthesis parameter after the PCM input. If optional_synthesis is null the encoder will be able to save some computation. If optional_synthesis is non-null if will be used to write the encoder's expectation of the decoder's output. Synthesis may alias the input pcm, so calling the encoder with the same buffer twice will achieve the old behavior. Remove 'restrict' from the CTL prototype.