This patch introduces a new macro: `either_n!` that generates the types
`Either{n}` where $3 \leq n \leq 26$. Manual implementations for
`Either3`, `Either4` and `Either5` are removed by this patch too.
The `either_n!` macro is quite classical. There is no particular
trick, except `count_idents!` which simply turns, e.g. `A, B, C` into
`3`. This macro is used by `either_n!` to implement the
`debug_assert!` inside `from_napi_value`.
