Lock down visibility of as much of the runtime as possible

[apple]

Lock down visibility of as much of the runtime as possible

[thomasvl]

Tagging as API changing since changing visibility adds/removes symbols.

[allevato]

Also, for things that have to be public as implementation details, move them into an extension of the Internal struct if possible (i.e., they're not generic or protocols).

[thomasvl]

I believe I've completely this, but before closing this out; @tbkka & @allevato can you both make a sweep of all the files/types to confirm what is public makes sense to you?

[tbkka]

I've gone through a grep public of the library code and it looks pretty good.

[allevato]

Ditto; I think we're done here for 1.0.