Create encoding module and exposing EncodingType

[nekevss]

This PR creates a proper encoding module in ixdtf and exposes the EncodingType trait to allow wrapping parsers to be generic on EncodingType.

[Manishearth]

This is a util that is easy to make breaking changes to, so I am comfortable stabilizing such an API for now. It's not necessarily the best API for this, but it's ... okay.

@sffc , thoughts?

[Manishearth]

I guess because it's sealed we should doc(hidden) the methods?

[nekevss]

I can doc(hidden) the methods. I don't see a reason for someone to be implementing the trait (hence why it's sealed).

Is there a general preference towards what the API should be for this? The main goal here is to expose the trait so that a parser that wraps the IxdtfParser can take advantage of the trait rather than needing an enum for Utf16 and Utf8.

[Manishearth]

Is there a general preference towards what the API should be for this?

No, just that I'm noting that we haven't thought too deeply about this. doc(hidden) lets us not have to worry about this.