Construct flow

[Fishrock123]

Some kind of construct flow would be very useful for a couple of significant reasons:

• Presently resources must either be opened upon constructor call, or upon "first pull".
  • The former presents async timing issues
  • The latter is complicated and messy
• It would be useful to pass buffer allocation hits out-of-flow (Buffer allocation hints #52)

Arguments for doing it all inline could be getting pretty long (and very variable), not even counting the first point:

pull(status_type, error, buffer, size, offset)

Maybe?

Idk, maybe separating this all out into multiple flows would be better, similar to Streams3 but just sans the dreaded EventEmitter.

• (sink calls) -> (source calls)
• construct(...) -> ready(...)
• pull(...) -> give(...)
• destroy(error) -> destroy(error)

Very related to nodejs/node#29314

[ronag]

I'll chime in a link to related PR in Node nodejs/node#29656.