[ruby-core:98958] [Ruby master Feature#16987] Enumerator::Lazy vs Array methods

Issue #16987 has been reported by zverok (Victor Shepelev).

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Feature #16987: Enumerator::Lazy vs Array methods
https://bugs.ruby-lang.org/issues/16987

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
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Context: Early in Ruby's life, it was made a design decisions for our enumerations to be greedy (immediately executed on method chaining). Sometimes it is not practical (e.g. 2mln strings array -- drop comments, split into fields, find first 10 whose field 2 is equal search value), so one either resorts to non-idiomatic "do everything in one `each` block", or to `Enumerable#lazy`. There are three problems with latter:
1. It is much less known
2. It is said to be _almost always_ slow than non-lazy, and therefore not recommended
3. It lacks some methods that are often necessary in processing large data chunks.

I want to discuss (3) here. Examples of methods `Enumerator::Lazy` would better, but actually don't have: `#flatten`, `#product`, `#compact`. One might notice it all methods of the Array, not Enumerable, but 
1. probably they _should_ be present in `Enumerable` (none of them require anything besides `#each` to function)
2. they are definitely useful to lazily process large sequences.



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