Encapsulate codec over Vec<u8> and slices

[tisonkun]

On a separate note, re: removing dependencies. What do we feel about byteorder?

I think we can have our own reiml perhaps in mod serde or find a better name. We can optimize our use cases over Vec<u8> and &[u8], while both BE and LE are needed - we still use BigEndian somewhere, like recognizing TDigests' reference format.

Let me track this task and try to implement it this week.

Originally posted by @tisonkun in #36 (comment)

[tisonkun]

Defined SketchBytes and SketchSlice in #49.

We may later have SketchReader<R: io::Read> and SketchWriter<R: io::Write>, especially when considering streaming IO. But we can review the requirement at that moment.