Bug 220519

Some type encoders are implemented in Encoder class Most type encoders for IPC serialization are implemented in the type (T::encode(Encoder&)) or in ArgumentCoder<T> template. However, arithmetic types and enums are implemented in Encoder. This makes it hard to implement other types of Encoders, since this code should be somehow duplicated or extracted redundantly to a base template. Instead, it would be consistent if all the serialization would be in in the T::encode or ArgumentCoder<T>::encode. This would be important for WebGL IPC, where the stream serialization would in general use the base serialization code, but some details of the Encoder would need to differ.