Create a plugin for straightforward communication with Elixir code outside of pipelines

[mat-hek]

The plugin should contain elements that would

• Send/receive buffers, events and stream formats to/from Elixir processes
• Possibly allow producing/consuming Elixir enumerables, for example

input = Stream.cycle([1, 2, 3])
output = Membrane.Elixir.Stream.new()

p = Membrane.RCPipeline.start_link!()
Membrane.RCPipeline.exec_actions(spec:
  child(%Membrane.Elixir.Source{input: input})
  |> child(%Membrane.Elixir.Sink{output: output})
)

output
|> Stream.map(& &1 + 1)
|> Enum.take(3)
# => [2, 3, 4]

Possibly it should end up in the core.