[WIP] Add concurrent processing for mixing samples

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Concurrent processor usage for mixing</issue_title>
<issue_description>Especially in a situation where the output is being written directly to stdout, it would be optimal to be able to distribute the load of mathematical operations to multiple processors.

Examine the sample.OutNext() binding:

// OutNext to mix the next sample for all channels, in []float64
func OutNext() []Value {
    return outNextCallback()
}

If we assign some number n to represent the number of concurrent mix processes, then usage of this sample.OutNext() could be batched such that instead of retrieving one sample at a time, the samples are retrieved in a single map-reduce sweep of n goroutines performing the math. Ergo, the final function signature might look more like:

// OutNextConcurrent() to concurrently mix the next samples for all channels, in []float64
func OutNextConcurrent() []Sample {
    // TODO: map-reduce goroutines of n mix-samples
}

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Concurrent processor usage for mixing #53

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