Initial proposal for Swift Backtracing API (#2140)

* Initial proposal for Swift Backtracing API

This SE proposal specifies a straightforward API that allows user code to
obtain and symbolicate backtraces.  It is not intended to be used from
signal handlers or to implement a crash catcher (the runtime already
includes code for that), but may be useful for capturing a backtrace
during normal execution, for instance while debugging a problem with some
code.

* Fix pitch link.

* Updated following pre-review comments.

Changed to using a non-concrete Address type, and to make the frames
member variables be a sequence rather than an array.

* Mention that this will exist in a module called Runtime.

* Update proposals/0419-backtrace-api.md

* Rename nnnn-backtrace-api.md to 0419-backtrace-api.md

* Update 0419-backtrace-api.md

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Co-authored-by: Stephen Canon <[email protected]>

Author: al45tair