feat: either_or combinator

[geovie]

This is an idea that came out of this feature request.

It adds an EitherOr trait providing a either_or method which can be used to "map" over bool, Option and Result types in a similar manner as e.g. Option.map_or_else.

With this there is yet another way to remove the need for to_any or to manually specify the Either::Left/Right:
e.g.

if let Some(key) = phantom_wallet_adapter.get().public_key() {
  view!{ ... }.into_any()
} else {
  view!{ ... }.into_any()
}

could be then written as

phantom_wallet_adapter.get().public_key().either_or(|key| view!{ ... }, |_| view!{ ... });

[gbj]

Thanks! I think this is nice.