remove cfg_attr for serde feature

[ralexstokes]

I had originally intended to make the serde feature optional, and gated the derive(serde::{Serialize,Deserialize}) attributes behind it, like this

ethereum-consensus/ethereum-consensus/src/altair/beacon_block.rs

Line 11 in 8c204e1

#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]

But it turns out this conditional attribute would also need to apply to each serde directive as well, like this

ethereum-consensus/ethereum-consensus/src/altair/beacon_block.rs

Line 44 in 8c204e1

#[serde(with = "crate::serde::as_string")]

otherwise, the crate does not compile without the serde feature. cf. #191 and #167

in lieu of adding a lot of code to what is already a bit of a noisy codebase (with all the const generics), I think at the moment I favor just ungating the derive(serde::*) directives for now and just require serde

while it does mean pulling in more crates than could otherwise be possible, serde is a common one and is usually in the closure of whatever crates a consumer of this library would already be using, so for most use cases it wouldn't matter. and it doesn't stop us from supporting no-std as serde has a capability for this as well.

so this issue will be closed once we have made the edits to remove all cfg_attr(serde) attributes.

[leruaa]

Hi, I can take this on :)

[ralexstokes]

fixed in #274