Macro ssz::impl_encode_via_from

macro_rules! impl_encode_via_from {
    ($impl_type: ty, $from_type: ty) => { ... };
}

Implements Encode for $impl_type using an implementation of From<$impl_type> for $from_type.

In effect, this allows for easy implementation of Encode for some type that implements a From conversion into another type that already has Encode implemented.