Add EncodingContext

Instead of users passing the format to (de)serializer API, they now pass a context that has other parameters as well:

• n_bytes_before: This enables values to be serialized/deserialized to/from the middle of a message. Hopefully this can later be used to create API to read only specific parts of a very large array.
• ByteOrder type parameter: This enables users to only specify the ByteOrder trait once when they create the context and then subsequently just use the generic (de)serializer API w/o having to specify the ByteOrder again.