generatorFlatMap / generatorFlat

[E-gy]

In a spiritual follow-up to #18, it would be nice to have flattening operations on async generators.

asyncGeneratorFlat would transform AsyncGenerator<AsyncGenerator<T, ...>> into AsyncGenerator<T>, and asyncGeneratorFlatMap (arguably more powerful) would iterate over AsyncGenerator<A, ...> with mapper function (A, ...) => AsyncGenerator<B, ...> and produce AsyncGenerator<B, ...>.

This can be especially powerful with a similar ordered: false flag, allowing for earliest-first yield at nested levels - a behavior that is not achievable via flat (nor reduction via trivial chaining of produced generators).

Those functions can be useful when aggregating 2nd layer nested data mapped 1:N to 1st layer which is also mapped 1:N to the outer data, in an asynchronous iterable manner (combine that with gRPC streams or websockets, and you can see how it can be used on the web). For example, in a data structure that lists boxes stored in every room in one data source, and the rooms in a house in a different data source, asyncGeneratorFlatMap can be used to obtain an unordered stream of all boxes in a house (presumably the 'stored in' and 'part of' relations are stored as distinct elements for every pair, with additional metadata, and retrieving them as a stream individually is more efficient than retrieving the whole block).