implement `fork( closure0, closure1,... )` and `select<1,2,3>()` adaptors

[ericniebler]

snd | fork( closure0, closure1 )

would be like:

just(split(snd)) 
  | let_value([](auto snd) {
      return when_all(snd | closure0, snd | closure1);
    })

It could be useful in tandem with a no-op pass_through adaptor and a select<I,J,K> adaptor that forwards only the I-th, J-th, K-th value. Then we could do something like:

snd | fork( pass_through, select<1,2,3> | then(f) )