[WIP] Shared string APIs

[allevato]

Floating an early sketch of some shared String APIs to improve interop code.

Still some open questions, like:

• Should this take advantage of ContiguousCollection instead of taking pointers directly (probably)
• Do we need a method to force a shared string into a native one, thus copying it (probably)

cc @milseman

[milseman]

This looks fine to me. I guess the other option would be to form a foreign string, such as a sliced NSString subclass.

[milseman]

This isn't small-string correct, but small substrings will form small strings, so eager copy is even faster than using __SharedStringStorag. What we'd want to do is check for a contiguous string with a stable pointer.

[karwa]

ping @milseman @allevato

Are there any plans to move this along with a pitch? Would you mind if I wrote one?

[allevato]

@karwa I don't know what @milseman 's plans are, but I haven't had free time lately to work on this so if you'd like to run with it, feel free!

[shahmishal]

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

• How to change the base branch: (Link)
• More detail about the branch update: (Link)