Port gen_stage & flow actor data processing libraries to Gleam

[lpil]

https://github.com/elixir-lang/gen_stage

https://github.com/dashbitco/flow

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https://discord.com/channels/768594524158427167/1308359897884397578/1308363351511928833
https://discord.com/channels/768594524158427167/1308359897884397578/1308382001526472765

https://hexdocs.pm/flow/Flow.html
https://hexdocs.pm/gen_stage/GenStage.html
https://hexdocs.pm/broadway/introduction.html