figure out better stream abstractions

for an explanation of this module see the intro here: https://github.com/maxogden/open-prs#open-prs

main plumbing code is here: https://github.com/maxogden/open-prs/blob/master/index.js#L12-L45a

things I want to do better:

• find a good module for doing multiple readables pipe -> single writable

(e.g. merging streams) https://github.com/maxogden/open-prs/blob/master/index.js#L40-L43

• find a way to do better parallelism

take this stream for example: https://github.com/maxogden/open-prs/blob/master/index.js#L26-L38

it is a single stream that spawns many child streams and emits their data through itself.

I want to parallelize it, but doing so would require tracking all parallel tasks so that the parent stream knew when to end, which will add a bunch of code. maybe theres a module that handles this nicely?

Start with the module introduction and the plumbing in index.js lines 12-45, especially the stream sections at lines 26-38 and 40-43. Investigate how multiple readables could feed one writable and how parallel child streams would signal completion. Done would require an agreed abstraction or module choice and a defined approach for parallel task tracking, but the issue does not specify acceptance criteria.