init accessors. (#2042)

* Add proposal for init accessors.

* Init accessors suggestions (#2)

* Expand description of accesses and give a "dictionary storage" example

* Make the name of the `newValue` parameter configurable

Be consistent with set/willSet/didSet in making the parameter name
optional and overridable.

* A few clarifications regarding init accessor signatures

* Clarify the rules for definite initialization

Expand the examples and show the case where virtual and stored-property
initialization collide.

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Co-authored-by: Holly Borla <[email protected]>

* Address outstanding proposal comments.

* Update the syntax for init accessor effects based on pitch feedback.

* Update the first init accessor example to use a custom parameter name.

* Add an acknowledgments section for the init accessors proposal.

* Update proposals/NNNN-init-accessors.md

Co-authored-by: Frederick Kellison-Linn <[email protected]>

* Remove the phrase "virtual initialization".

* Editorial changes.

* Update the Alternatives Considered section with more syntax suggestions from
the pitch thread.

* Add more commentary on member-wise initializers.

* Add a DI example where a property is not initialized on all paths.

* Init accessors is SE-0400

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Co-authored-by: Doug Gregor <[email protected]>
Co-authored-by: Frederick Kellison-Linn <[email protected]>
Co-authored-by: Freddy Kellison-Linn <[email protected]>

Author: 3 people