Add implementation of Decode for Box<str> and Box<[T]>

[mversic]

Closes #564

I can't think of a way to implement Decode for all unsized types

[bkchr]

Can you check the already existing Box Decode implementation. It should be in the same way as this implementation.

[mversic]

Do you think that decoded String will have some excess capacity or will it have capacity == len?

[mversic]

I've taken a bit different route here

[mversic]

should I maybe use WrapperTypeDecode like this:

impl WrapperTypeDecode for Box<str> {
	type Wrapped = String;

	fn decode_wrapped<I: Input>(input: &mut I) -> Result<Self, Error> {
		// Guaranteed to create a Vec with capacity == len
		let vec = Vec::from(Box::<[u8]>::decode(input)?);
		// Guaranteed not to reallocate the vec, only transmute to String
		let str = String::from_utf8(vec).map_err(|_| "Invalid utf8 sequence")?;

		// At this point we have String with capacity == len,
		// therefore String::into_boxed_str will not reallocate
		Ok(str.into_boxed_str())
	}
}

[mversic]

can I get a response here?

[mversic]

hi, this PR should be ready for merge. Can someone approve it now?

[nazar-pc]

Would be great to get an impl for Arc<[T]> as well (see #633)

[mversic]

Would be great to get an impl for Arc<[T]> as well (see #633)

I don't want to complicate this PR further as it's not being reviewed as it is.
I'd be happy to implement Rc<[T]>/Arc<[T]> in a separate PR.