[Suggestion] Add Swift-Collections library to stdlib

[xwu (Xiaodi Wu)]

From the announcement post:

Relation to the Swift Standard Library

It’s our ambition for the standard library to include a rich, pragmatic set of general-purpose data structures.

Similar to packages like Swift Numericsand Swift Algorithms, an important goal of the Collections package is to serve as a (relatively) low-friction proving ground for new data structure implementations, before they become ready to be proposed as official library additions through the regular Swift Evolution process.

The experience we gain using these constructs in package form will inform the eventual review discussion. It will also provide us an opportunity to correct any design issues before they get etched into stone.

The Collections package is, in part, a recognition of the challenges involved in contributing new data structures to Swift. Because the standard library is ABI-stable, a lot of time must be spent thinking about which parts of a data structure are going to be @frozen and which aren’t, and which methods should be @inlinable and touch those frozen internals.

Accordingly, the Collections package is not just a set of data structures. It is also a watering hole for contributors who want to learn more about the dark art of ABI design and a sophisticated toolkit to help meet the high standards of correctness and efficiency demanded of data structures.