Add skip()

[bkchr]

The Decode trait should get a method skip(input: &mut Input) to skip an encoded object.

[gui1117]

we do agree that skipping an encoded object shouldn't check that the object is valid.

So if someone works on untrusted data, skipping some part of the data to get the wanted part can succeed even if the data will actually fail to decode.

[bkchr]

We don't really check if an object is valid?

But yeah, normally we just skip the amount of expected bytes.

[gui1117]

Maybe we should also introduce a new function:

/// Can return the length of the type if fixed.
fn length() -> Option<usize> { None }

this would be used to optimized implementation of Vec<T>::skip and some other types

if let Some(size) = T::length {
  // decode compact and skip size*compact
}

At least we could put this new method in decode with default to None and improve on it later.

[gui1117]

skip is now part of Decode trait. I think we can close this issue in favor of #244
The idea is to release 2.0 soon so it can goes into next substrate release