Add Iterable to support streaming

[huyuncong]

Description

Add streaming utilities as Iterable into the std library.
The struct supports future developments for streaming.

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[weikengchen]

Suggested change

	/// Returns the iterator associated to the current instance.
	/// Returns the iterator associated to the current instance.

[weikengchen]

This sounds like the entire Iterable trait is not necessary, and it is the duty for Gemini to not use Iterable but instead to use the standard library one.

What is the main difference that Iterable provides but not by IntoIterator?

[mmaker]

Fair point: the main difference is that we need to iter over the same stream multiple times, by design.
This means that already we require IntoIterator + Copy or IntoIterator + Clone if we want to go down that avenue.

From there, it's pretty much the same, except that now to get the length of a stream it becomes a bit more verbose, and the length function might disagree with the actual length (this is needed for instance when streaming a matrix).
Here's a bit of an overview of how the code might differ: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c7d9d2fca7d1d308601bd5b32d770409

To be fair, on the one hand, the downside is that Iterable::iter will have to copy all relevant data from the initial structure (until https://github.com/rust-lang/rfcs/blob/master/text/1598-generic_associated_types.md hits the streets). On the other hand, for the long-term vision we have a trait that can be extended with other functions on the top of normal iterables, and that we skip having to type things like vec.into_iter().len().