Implement more traits for EasyBuf

[Yamakaky]

Clone, Deref, Into<Vec<u8>> would be a start.

[tailhook]

I think also Index<..> would be cool. Because drain_to is very-limited.

[Yamakaky]

Deref<[u8]> would be sufficient, I think?

[tailhook]

No, I mean slicing should return EasyBuf not &[u8], because EasyBuf has a static lifetime, so can be a part of a return value of the protocol parser (and EasyBuf is specifically designed not to make a clone of the underlying data for that, it's how drait_to works)

[Yamakaky]

Oh, yeah

[alexcrichton]

The intention was to be very conservative for the initial iteration that we could tack on more functionality later, but I couldn't mind adding a few more common trait impls and such. I'd prefer to avoid Deref for now to avoid conflicts with the slice API, but other common traits should be good to go.

[aturon]

cc #128

[manuel-woelker]

@tailhook I was looking at implementing Index for EasyBuf, with Output = EasyBuf. But it seems index
returns &Self::output. That means my naive approach - creating a new EasyBuf instance to return - won't work since it does not live long enough. Any pointers on how to proceed? Would this require IndexGet/IndexMove?

[tailhook]

@manuel-woelker, hm, looks like we can't do that :(. The best way I can see is to have a method x.slice(a..b).

[alexcrichton]

Done in https://github.com/alexcrichton/tokio-io, the migration of this crate will be tracked by #61