Adds a from_fn stream implementation

[montekki]

Hope the example is not too ugly.

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Ref: #129
Stdlib: https://doc.rust-lang.org/std/iter/fn.from_fn.html

[yoshuawuyts]

Related #130 -- we closed that PR because async closures aren't really useful right now. But I guess it's "later" now, and with #262 and #279 we're already introducing several functions relying on async closures, so this might feel less out of place.