v.NEXT: Support for async generators (Stream protocol)

For your next version. (I read that there is a module-based rewrite in the works! 👍 )

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- iCal feeds can be quite large
- iCal recurrence computation can get quite lengthy

_Anything_ that involves a potentially long read or computation would benefit greatly from the use of async generators. Parsing and computing recurrence rules are the top two candidates that spring to mind.

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[This excellent post on 2ality](https://2ality.com/2019/11/nodejs-streams-async-iteration.html) explains the benefits in far greater detail and clarity than I ever could.

The blog post was also the progenitor of [the equivalent section in his book](https://exploringjs.com/impatient-js/ch_async-iteration.html#async-generators) (which presents very similar info, but _slightly_ differently; you may prefer one over the other).

Start by reviewing the project’s parsing and recurrence-computation entry points, then compare their current behavior with the linked async-iteration resources. The issue’s goal is to support async generators through the Stream protocol for potentially long reads and computations, especially iCal parsing and recurrence calculation.

Written by the indexing model from the issue text.