Added spaced function, similar to metered but waiting

[eugeniasimich]

While working with the library I was kind of expecting metered to behave like the function I add in this PR.
In short, the difference between spaced and metered is analogous to the one between fixedDelay and fixedRate. I also think having both makes more clear what metered does.
Also decided not to have two versions of it (like metered and meteredStartsImmediately) but that's just an opinion.
I can add documentation as well. What do you think?

[diesalbla]

Also decided not to have two versions of it (like metered and meteredStartsImmediately) but that's just an opinion.

Perhaps a Boolean parameter startsImmediately with a default choice (false) would do.

I can add documentation as well. What do you think?

I can suggest some changes

[diesalbla]

So, while writing that documentation suggestion, it just came to me...

How would this combinator differ from just doing the following?

  stream.evalTap(_ => Temporal[F].sleep(delay))

(trying to remember the Temporal api as I can)

As described in the long comment, the purpose of metered is to make those delays aware and adjusted to other delays in the whole stream, either by the source stream or by the consumer. If this action is to be independent of that context, the evalTap could do?

[eugeniasimich]

The difference between spaced and evalTapping a delay is that, for finite streams, spacing the stream won't perform a delay after the last element.

[eugeniasimich]

@diesalbla I've added a commit to address you suggestion

[diesalbla]

Suggested change

	* The resulting stream emits the same elements from `this` stream,
	* but it emits them in singleton chunks. Between each singleton chunk
	* (each element) it introduces a pause that takes the `delay` duration.
	* The resulting stream emits the same elements from `this` stream,
	* but split into singleton chunks. Between each chunk (element) it
	* adds a pause of a fixed `delay` duration.

(slight correction)

[diesalbla]

@eugeniasimich Before merging, could you squash all commits into one?

[eugeniasimich]

done!