EntitySet does not have feature="enable-serde"

[djmarcin]

Feature

EntitySet does not have the following annotation, despite appearing to be mostly a wrapper around a CompoundBitSet, which does. I'm hoping this is just an oversight?

#[derive(Clone, Default, PartialEq, Eq)]
#[cfg_attr(
    feature = "enable-serde",
    derive(serde_derive::Serialize, serde_derive::Deserialize)
)]

Benefit

Allows EntitySet to be used in Serializable data structures like PrimaryMap/SecondaryMap/etc.

Implementation

Add the missing annotation.

Alternatives

n/a