Add io::Read::read_le and io::Read::read_be

[joshtriplett]

These functions make it easy to read a fixed-size type as little-endian
or big-endian. They're trivial wrappers around the combination of
io::Read::read_array and T::from_le_bytes/T::from_be_bytes.

The implementation uses a sealed trait FromEndianBytes. That trait is
currently in std and accepts a &mut io::Read. Once we can use
associated consts in the types of method parameters, we can change this
trait to have from_le_bytes and from_be_bytes methods, move it to
core, and make it public.

---

As discussed at RustWeek.

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[Darksonn]

Just a warning: My experience from having methods similar to these in the bytes crate is that you will receive endless requests to add more and more variations of them. Just look at how many methods Buf has at this point.

[joshtriplett]

@Darksonn I'm hoping that having generic methods (such that we don't need a separate method per return type) will avoid the combinatorial explosion. We also don't need the try_ variants because these are on Read and already always return a result.

[clarfonthey]

Is there any other discussions that might be relevant to link about this? I figure there isn't an ACP, but I saw that a bunch of RustWeek meetings seemed to have involved various documents people made, and wonder if there's an associated one for this.

[Darksonn]

Yes, the trait does help somewhat.